The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_sglist.c

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    1 /*-
    2  * Copyright (c) 2008 Yahoo!, Inc.
    3  * All rights reserved.
    4  * Written by: John Baldwin <jhb@FreeBSD.org>
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice, this list of conditions and the following disclaimer.
   11  * 2. Redistributions in binary form must reproduce the above copyright
   12  *    notice, this list of conditions and the following disclaimer in the
   13  *    documentation and/or other materials provided with the distribution.
   14  * 3. Neither the name of the author nor the names of any co-contributors
   15  *    may be used to endorse or promote products derived from this software
   16  *    without specific prior written permission.
   17  *
   18  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   19  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   20  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   21  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   22  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   23  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   24  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   25  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   26  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   27  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   28  * SUCH DAMAGE.
   29  */
   30 
   31 #include <sys/cdefs.h>
   32 __FBSDID("$FreeBSD: stable/10/sys/kern/subr_sglist.c 297059 2016-03-20 05:01:40Z np $");
   33 
   34 #include <sys/param.h>
   35 #include <sys/kernel.h>
   36 #include <sys/bio.h>
   37 #include <sys/malloc.h>
   38 #include <sys/mbuf.h>
   39 #include <sys/proc.h>
   40 #include <sys/sglist.h>
   41 #include <sys/uio.h>
   42 
   43 #include <vm/vm.h>
   44 #include <vm/vm_page.h>
   45 #include <vm/pmap.h>
   46 #include <vm/vm_map.h>
   47 
   48 #include <sys/ktr.h>
   49 
   50 static MALLOC_DEFINE(M_SGLIST, "sglist", "scatter/gather lists");
   51 
   52 /*
   53  * Convenience macros to save the state of an sglist so it can be restored
   54  * if an append attempt fails.  Since sglist's only grow we only need to
   55  * save the current count of segments and the length of the ending segment.
   56  * Earlier segments will not be changed by an append, and the only change
   57  * that can occur to the ending segment is that it can be extended.
   58  */
   59 struct sgsave {
   60         u_short sg_nseg;
   61         size_t ss_len;
   62 };
   63 
   64 #define SGLIST_SAVE(sg, sgsave) do {                                    \
   65         (sgsave).sg_nseg = (sg)->sg_nseg;                               \
   66         if ((sgsave).sg_nseg > 0)                                       \
   67                 (sgsave).ss_len = (sg)->sg_segs[(sgsave).sg_nseg - 1].ss_len; \
   68         else                                                            \
   69                 (sgsave).ss_len = 0;                                    \
   70 } while (0)
   71 
   72 #define SGLIST_RESTORE(sg, sgsave) do {                                 \
   73         (sg)->sg_nseg = (sgsave).sg_nseg;                               \
   74         if ((sgsave).sg_nseg > 0)                                       \
   75                 (sg)->sg_segs[(sgsave).sg_nseg - 1].ss_len = (sgsave).ss_len; \
   76 } while (0)
   77 
   78 /*
   79  * Append a single (paddr, len) to a sglist.  sg is the list and ss is
   80  * the current segment in the list.  If we run out of segments then
   81  * EFBIG will be returned.
   82  */
   83 static __inline int
   84 _sglist_append_range(struct sglist *sg, struct sglist_seg **ssp,
   85     vm_paddr_t paddr, size_t len)
   86 {
   87         struct sglist_seg *ss;
   88 
   89         ss = *ssp;
   90         if (ss->ss_paddr + ss->ss_len == paddr)
   91                 ss->ss_len += len;
   92         else {
   93                 if (sg->sg_nseg == sg->sg_maxseg)
   94                         return (EFBIG);
   95                 ss++;
   96                 ss->ss_paddr = paddr;
   97                 ss->ss_len = len;
   98                 sg->sg_nseg++;
   99                 *ssp = ss;
  100         }
  101         return (0);
  102 }
  103 
  104 /*
  105  * Worker routine to append a virtual address range (either kernel or
  106  * user) to a scatter/gather list.
  107  */
  108 static __inline int
  109 _sglist_append_buf(struct sglist *sg, void *buf, size_t len, pmap_t pmap,
  110     size_t *donep)
  111 {
  112         struct sglist_seg *ss;
  113         vm_offset_t vaddr, offset;
  114         vm_paddr_t paddr;
  115         size_t seglen;
  116         int error;
  117 
  118         if (donep)
  119                 *donep = 0;
  120         if (len == 0)
  121                 return (0);
  122 
  123         /* Do the first page.  It may have an offset. */
  124         vaddr = (vm_offset_t)buf;
  125         offset = vaddr & PAGE_MASK;
  126         if (pmap != NULL)
  127                 paddr = pmap_extract(pmap, vaddr);
  128         else
  129                 paddr = pmap_kextract(vaddr);
  130         seglen = MIN(len, PAGE_SIZE - offset);
  131         if (sg->sg_nseg == 0) {
  132                 ss = sg->sg_segs;
  133                 ss->ss_paddr = paddr;
  134                 ss->ss_len = seglen;
  135                 sg->sg_nseg = 1;
  136         } else {
  137                 ss = &sg->sg_segs[sg->sg_nseg - 1];
  138                 error = _sglist_append_range(sg, &ss, paddr, seglen);
  139                 if (error)
  140                         return (error);
  141         }
  142         vaddr += seglen;
  143         len -= seglen;
  144         if (donep)
  145                 *donep += seglen;
  146 
  147         while (len > 0) {
  148                 seglen = MIN(len, PAGE_SIZE);
  149                 if (pmap != NULL)
  150                         paddr = pmap_extract(pmap, vaddr);
  151                 else
  152                         paddr = pmap_kextract(vaddr);
  153                 error = _sglist_append_range(sg, &ss, paddr, seglen);
  154                 if (error)
  155                         return (error);
  156                 vaddr += seglen;
  157                 len -= seglen;
  158                 if (donep)
  159                         *donep += seglen;
  160         }
  161 
  162         return (0);
  163 }
  164 
  165 /*
  166  * Determine the number of scatter/gather list elements needed to
  167  * describe a kernel virtual address range.
  168  */
  169 int
  170 sglist_count(void *buf, size_t len)
  171 {
  172         vm_offset_t vaddr, vendaddr;
  173         vm_paddr_t lastaddr, paddr;
  174         int nsegs;
  175 
  176         if (len == 0)
  177                 return (0);
  178 
  179         vaddr = trunc_page((vm_offset_t)buf);
  180         vendaddr = (vm_offset_t)buf + len;
  181         nsegs = 1;
  182         lastaddr = pmap_kextract(vaddr);
  183         vaddr += PAGE_SIZE;
  184         while (vaddr < vendaddr) {
  185                 paddr = pmap_kextract(vaddr);
  186                 if (lastaddr + PAGE_SIZE != paddr)
  187                         nsegs++;
  188                 lastaddr = paddr;
  189                 vaddr += PAGE_SIZE;
  190         }
  191         return (nsegs);
  192 }
  193 
  194 /*
  195  * Allocate a scatter/gather list along with 'nsegs' segments.  The
  196  * 'mflags' parameters are the same as passed to malloc(9).  The caller
  197  * should use sglist_free() to free this list.
  198  */
  199 struct sglist *
  200 sglist_alloc(int nsegs, int mflags)
  201 {
  202         struct sglist *sg;
  203 
  204         sg = malloc(sizeof(struct sglist) + nsegs * sizeof(struct sglist_seg),
  205             M_SGLIST, mflags);
  206         if (sg == NULL)
  207                 return (NULL);
  208         sglist_init(sg, nsegs, (struct sglist_seg *)(sg + 1));
  209         return (sg);
  210 }
  211 
  212 /*
  213  * Free a scatter/gather list allocated via sglist_allc().
  214  */
  215 void
  216 sglist_free(struct sglist *sg)
  217 {
  218 
  219         if (sg == NULL)
  220                 return;
  221 
  222         if (refcount_release(&sg->sg_refs))
  223                 free(sg, M_SGLIST);
  224 }
  225 
  226 /*
  227  * Append the segments to describe a single kernel virtual address
  228  * range to a scatter/gather list.  If there are insufficient
  229  * segments, then this fails with EFBIG.
  230  */
  231 int
  232 sglist_append(struct sglist *sg, void *buf, size_t len)
  233 {
  234         struct sgsave save;
  235         int error;
  236 
  237         if (sg->sg_maxseg == 0)
  238                 return (EINVAL);
  239         SGLIST_SAVE(sg, save);
  240         error = _sglist_append_buf(sg, buf, len, NULL, NULL);
  241         if (error)
  242                 SGLIST_RESTORE(sg, save);
  243         return (error);
  244 }
  245 
  246 /*
  247  * Append the segments to describe a bio's data to a scatter/gather list.
  248  * If there are insufficient segments, then this fails with EFBIG.
  249  *
  250  * NOTE: This function expects bio_bcount to be initialized.
  251  */
  252 int
  253 sglist_append_bio(struct sglist *sg, struct bio *bp)
  254 {
  255         struct sgsave save;
  256         vm_paddr_t paddr;
  257         size_t len, tlen;
  258         int error, i, ma_offs;
  259 
  260         if ((bp->bio_flags & BIO_UNMAPPED) == 0) {
  261                 error = sglist_append(sg, bp->bio_data, bp->bio_bcount);
  262                 return (error);
  263         }
  264 
  265         if (sg->sg_maxseg == 0)
  266                 return (EINVAL);
  267 
  268         SGLIST_SAVE(sg, save);
  269         tlen = bp->bio_bcount;
  270         ma_offs = bp->bio_ma_offset;
  271         for (i = 0; tlen > 0; i++, tlen -= len) {
  272                 len = min(PAGE_SIZE - ma_offs, tlen);
  273                 paddr = VM_PAGE_TO_PHYS(bp->bio_ma[i]) + ma_offs;
  274                 error = sglist_append_phys(sg, paddr, len);
  275                 if (error) {
  276                         SGLIST_RESTORE(sg, save);
  277                         return (error);
  278                 }
  279                 ma_offs = 0;
  280         }
  281         return (0);
  282 }
  283 
  284 /*
  285  * Append a single physical address range to a scatter/gather list.
  286  * If there are insufficient segments, then this fails with EFBIG.
  287  */
  288 int
  289 sglist_append_phys(struct sglist *sg, vm_paddr_t paddr, size_t len)
  290 {
  291         struct sglist_seg *ss;
  292         struct sgsave save;
  293         int error;
  294 
  295         if (sg->sg_maxseg == 0)
  296                 return (EINVAL);
  297         if (len == 0)
  298                 return (0);
  299 
  300         if (sg->sg_nseg == 0) {
  301                 sg->sg_segs[0].ss_paddr = paddr;
  302                 sg->sg_segs[0].ss_len = len;
  303                 sg->sg_nseg = 1;
  304                 return (0);
  305         }
  306         ss = &sg->sg_segs[sg->sg_nseg - 1];
  307         SGLIST_SAVE(sg, save);
  308         error = _sglist_append_range(sg, &ss, paddr, len);
  309         if (error)
  310                 SGLIST_RESTORE(sg, save);
  311         return (error);
  312 }
  313 
  314 /*
  315  * Append the segments that describe a single mbuf chain to a
  316  * scatter/gather list.  If there are insufficient segments, then this
  317  * fails with EFBIG.
  318  */
  319 int
  320 sglist_append_mbuf(struct sglist *sg, struct mbuf *m0)
  321 {
  322         struct sgsave save;
  323         struct mbuf *m;
  324         int error;
  325 
  326         if (sg->sg_maxseg == 0)
  327                 return (EINVAL);
  328 
  329         error = 0;
  330         SGLIST_SAVE(sg, save);
  331         for (m = m0; m != NULL; m = m->m_next) {
  332                 if (m->m_len > 0) {
  333                         error = sglist_append(sg, m->m_data, m->m_len);
  334                         if (error) {
  335                                 SGLIST_RESTORE(sg, save);
  336                                 return (error);
  337                         }
  338                 }
  339         }
  340         return (0);
  341 }
  342 
  343 /*
  344  * Append the segments that describe a single user address range to a
  345  * scatter/gather list.  If there are insufficient segments, then this
  346  * fails with EFBIG.
  347  */
  348 int
  349 sglist_append_user(struct sglist *sg, void *buf, size_t len, struct thread *td)
  350 {
  351         struct sgsave save;
  352         int error;
  353 
  354         if (sg->sg_maxseg == 0)
  355                 return (EINVAL);
  356         SGLIST_SAVE(sg, save);
  357         error = _sglist_append_buf(sg, buf, len,
  358             vmspace_pmap(td->td_proc->p_vmspace), NULL);
  359         if (error)
  360                 SGLIST_RESTORE(sg, save);
  361         return (error);
  362 }
  363 
  364 /*
  365  * Append the segments that describe a single uio to a scatter/gather
  366  * list.  If there are insufficient segments, then this fails with
  367  * EFBIG.
  368  */
  369 int
  370 sglist_append_uio(struct sglist *sg, struct uio *uio)
  371 {
  372         struct iovec *iov;
  373         struct sgsave save;
  374         size_t resid, minlen;
  375         pmap_t pmap;
  376         int error, i;
  377 
  378         if (sg->sg_maxseg == 0)
  379                 return (EINVAL);
  380 
  381         resid = uio->uio_resid;
  382         iov = uio->uio_iov;
  383 
  384         if (uio->uio_segflg == UIO_USERSPACE) {
  385                 KASSERT(uio->uio_td != NULL,
  386                     ("sglist_append_uio: USERSPACE but no thread"));
  387                 pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace);
  388         } else
  389                 pmap = NULL;
  390 
  391         error = 0;
  392         SGLIST_SAVE(sg, save);
  393         for (i = 0; i < uio->uio_iovcnt && resid != 0; i++) {
  394                 /*
  395                  * Now at the first iovec to load.  Load each iovec
  396                  * until we have exhausted the residual count.
  397                  */
  398                 minlen = MIN(resid, iov[i].iov_len);
  399                 if (minlen > 0) {
  400                         error = _sglist_append_buf(sg, iov[i].iov_base, minlen,
  401                             pmap, NULL);
  402                         if (error) {
  403                                 SGLIST_RESTORE(sg, save);
  404                                 return (error);
  405                         }
  406                         resid -= minlen;
  407                 }
  408         }
  409         return (0);
  410 }
  411 
  412 /*
  413  * Append the segments that describe at most 'resid' bytes from a
  414  * single uio to a scatter/gather list.  If there are insufficient
  415  * segments, then only the amount that fits is appended.
  416  */
  417 int
  418 sglist_consume_uio(struct sglist *sg, struct uio *uio, size_t resid)
  419 {
  420         struct iovec *iov;
  421         size_t done;
  422         pmap_t pmap;
  423         int error, len;
  424 
  425         if (sg->sg_maxseg == 0)
  426                 return (EINVAL);
  427 
  428         if (uio->uio_segflg == UIO_USERSPACE) {
  429                 KASSERT(uio->uio_td != NULL,
  430                     ("sglist_consume_uio: USERSPACE but no thread"));
  431                 pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace);
  432         } else
  433                 pmap = NULL;
  434 
  435         error = 0;
  436         while (resid > 0 && uio->uio_resid) {
  437                 iov = uio->uio_iov;
  438                 len = iov->iov_len;
  439                 if (len == 0) {
  440                         uio->uio_iov++;
  441                         uio->uio_iovcnt--;
  442                         continue;
  443                 }
  444                 if (len > resid)
  445                         len = resid;
  446 
  447                 /*
  448                  * Try to append this iovec.  If we run out of room,
  449                  * then break out of the loop.
  450                  */
  451                 error = _sglist_append_buf(sg, iov->iov_base, len, pmap, &done);
  452                 iov->iov_base = (char *)iov->iov_base + done;
  453                 iov->iov_len -= done;
  454                 uio->uio_resid -= done;
  455                 uio->uio_offset += done;
  456                 resid -= done;
  457                 if (error)
  458                         break;
  459         }
  460         return (0);
  461 }
  462 
  463 /*
  464  * Allocate and populate a scatter/gather list to describe a single
  465  * kernel virtual address range.
  466  */
  467 struct sglist *
  468 sglist_build(void *buf, size_t len, int mflags)
  469 {
  470         struct sglist *sg;
  471         int nsegs;
  472 
  473         if (len == 0)
  474                 return (NULL);
  475 
  476         nsegs = sglist_count(buf, len);
  477         sg = sglist_alloc(nsegs, mflags);
  478         if (sg == NULL)
  479                 return (NULL);
  480         if (sglist_append(sg, buf, len) != 0) {
  481                 sglist_free(sg);
  482                 return (NULL);
  483         }
  484         return (sg);
  485 }
  486 
  487 /*
  488  * Clone a new copy of a scatter/gather list.
  489  */
  490 struct sglist *
  491 sglist_clone(struct sglist *sg, int mflags)
  492 {
  493         struct sglist *new;
  494 
  495         if (sg == NULL)
  496                 return (NULL);
  497         new = sglist_alloc(sg->sg_maxseg, mflags);
  498         if (new == NULL)
  499                 return (NULL);
  500         new->sg_nseg = sg->sg_nseg;
  501         bcopy(sg->sg_segs, new->sg_segs, sizeof(struct sglist_seg) *
  502             sg->sg_nseg);
  503         return (new);
  504 }
  505 
  506 /*
  507  * Calculate the total length of the segments described in a
  508  * scatter/gather list.
  509  */
  510 size_t
  511 sglist_length(struct sglist *sg)
  512 {
  513         size_t space;
  514         int i;
  515 
  516         space = 0;
  517         for (i = 0; i < sg->sg_nseg; i++)
  518                 space += sg->sg_segs[i].ss_len;
  519         return (space);
  520 }
  521 
  522 /*
  523  * Split a scatter/gather list into two lists.  The scatter/gather
  524  * entries for the first 'length' bytes of the 'original' list are
  525  * stored in the '*head' list and are removed from 'original'.
  526  *
  527  * If '*head' is NULL, then a new list will be allocated using
  528  * 'mflags'.  If M_NOWAIT is specified and the allocation fails,
  529  * ENOMEM will be returned.
  530  *
  531  * If '*head' is not NULL, it should point to an empty sglist.  If it
  532  * does not have enough room for the remaining space, then EFBIG will
  533  * be returned.  If '*head' is not empty, then EINVAL will be
  534  * returned.
  535  *
  536  * If 'original' is shared (refcount > 1), then EDOOFUS will be
  537  * returned.
  538  */
  539 int
  540 sglist_split(struct sglist *original, struct sglist **head, size_t length,
  541     int mflags)
  542 {
  543         struct sglist *sg;
  544         size_t space, split;
  545         int count, i;
  546 
  547         if (original->sg_refs > 1)
  548                 return (EDOOFUS);
  549 
  550         /* Figure out how big of a sglist '*head' has to hold. */
  551         count = 0;
  552         space = 0;
  553         split = 0;
  554         for (i = 0; i < original->sg_nseg; i++) {
  555                 space += original->sg_segs[i].ss_len;
  556                 count++;
  557                 if (space >= length) {
  558                         /*
  559                          * If 'length' falls in the middle of a
  560                          * scatter/gather list entry, then 'split'
  561                          * holds how much of that entry will remain in
  562                          * 'original'.
  563                          */
  564                         split = space - length;
  565                         break;
  566                 }
  567         }
  568 
  569         /* Nothing to do, so leave head empty. */
  570         if (count == 0)
  571                 return (0);
  572 
  573         if (*head == NULL) {
  574                 sg = sglist_alloc(count, mflags);
  575                 if (sg == NULL)
  576                         return (ENOMEM);
  577                 *head = sg;
  578         } else {
  579                 sg = *head;
  580                 if (sg->sg_maxseg < count)
  581                         return (EFBIG);
  582                 if (sg->sg_nseg != 0)
  583                         return (EINVAL);
  584         }
  585 
  586         /* Copy 'count' entries to 'sg' from 'original'. */
  587         bcopy(original->sg_segs, sg->sg_segs, count *
  588             sizeof(struct sglist_seg));
  589         sg->sg_nseg = count;
  590 
  591         /*
  592          * If we had to split a list entry, fixup the last entry in
  593          * 'sg' and the new first entry in 'original'.  We also
  594          * decrement 'count' by 1 since we will only be removing
  595          * 'count - 1' segments from 'original' now.
  596          */
  597         if (split != 0) {
  598                 count--;
  599                 sg->sg_segs[count].ss_len -= split;
  600                 original->sg_segs[count].ss_paddr =
  601                     sg->sg_segs[count].ss_paddr + split;
  602                 original->sg_segs[count].ss_len = split;
  603         }
  604 
  605         /* Trim 'count' entries from the front of 'original'. */
  606         original->sg_nseg -= count;
  607         bcopy(original->sg_segs + count, original->sg_segs, count *
  608             sizeof(struct sglist_seg));
  609         return (0);
  610 }
  611 
  612 /*
  613  * Append the scatter/gather list elements in 'second' to the
  614  * scatter/gather list 'first'.  If there is not enough space in
  615  * 'first', EFBIG is returned.
  616  */
  617 int
  618 sglist_join(struct sglist *first, struct sglist *second)
  619 {
  620         struct sglist_seg *flast, *sfirst;
  621         int append;
  622 
  623         /* If 'second' is empty, there is nothing to do. */
  624         if (second->sg_nseg == 0)
  625                 return (0);
  626 
  627         /*
  628          * If the first entry in 'second' can be appended to the last entry
  629          * in 'first' then set append to '1'.
  630          */
  631         append = 0;
  632         flast = &first->sg_segs[first->sg_nseg - 1];
  633         sfirst = &second->sg_segs[0];
  634         if (first->sg_nseg != 0 &&
  635             flast->ss_paddr + flast->ss_len == sfirst->ss_paddr)
  636                 append = 1;
  637 
  638         /* Make sure 'first' has enough room. */
  639         if (first->sg_nseg + second->sg_nseg - append > first->sg_maxseg)
  640                 return (EFBIG);
  641 
  642         /* Merge last in 'first' and first in 'second' if needed. */
  643         if (append)
  644                 flast->ss_len += sfirst->ss_len;
  645 
  646         /* Append new segments from 'second' to 'first'. */
  647         bcopy(first->sg_segs + first->sg_nseg, second->sg_segs + append,
  648             (second->sg_nseg - append) * sizeof(struct sglist_seg));
  649         first->sg_nseg += second->sg_nseg - append;
  650         sglist_reset(second);
  651         return (0);
  652 }
  653 
  654 /*
  655  * Generate a new scatter/gather list from a range of an existing
  656  * scatter/gather list.  The 'offset' and 'length' parameters specify
  657  * the logical range of the 'original' list to extract.  If that range
  658  * is not a subset of the length of 'original', then EINVAL is
  659  * returned.  The new scatter/gather list is stored in '*slice'.
  660  *
  661  * If '*slice' is NULL, then a new list will be allocated using
  662  * 'mflags'.  If M_NOWAIT is specified and the allocation fails,
  663  * ENOMEM will be returned.
  664  *
  665  * If '*slice' is not NULL, it should point to an empty sglist.  If it
  666  * does not have enough room for the remaining space, then EFBIG will
  667  * be returned.  If '*slice' is not empty, then EINVAL will be
  668  * returned.
  669  */
  670 int
  671 sglist_slice(struct sglist *original, struct sglist **slice, size_t offset,
  672     size_t length, int mflags)
  673 {
  674         struct sglist *sg;
  675         size_t space, end, foffs, loffs;
  676         int count, i, fseg;
  677 
  678         /* Nothing to do. */
  679         if (length == 0)
  680                 return (0);
  681 
  682         /* Figure out how many segments '*slice' needs to have. */
  683         end = offset + length;
  684         space = 0;
  685         count = 0;
  686         fseg = 0;
  687         foffs = loffs = 0;
  688         for (i = 0; i < original->sg_nseg; i++) {
  689                 space += original->sg_segs[i].ss_len;
  690                 if (space > offset) {
  691                         /*
  692                          * When we hit the first segment, store its index
  693                          * in 'fseg' and the offset into the first segment
  694                          * of 'offset' in 'foffs'.
  695                          */
  696                         if (count == 0) {
  697                                 fseg = i;
  698                                 foffs = offset - (space -
  699                                     original->sg_segs[i].ss_len);
  700                                 CTR1(KTR_DEV, "sglist_slice: foffs = %08lx",
  701                                     foffs);
  702                         }
  703                         count++;
  704 
  705                         /*
  706                          * When we hit the last segment, break out of
  707                          * the loop.  Store the amount of extra space
  708                          * at the end of this segment in 'loffs'.
  709                          */
  710                         if (space >= end) {
  711                                 loffs = space - end;
  712                                 CTR1(KTR_DEV, "sglist_slice: loffs = %08lx",
  713                                     loffs);
  714                                 break;
  715                         }
  716                 }
  717         }
  718 
  719         /* If we never hit 'end', then 'length' ran off the end, so fail. */
  720         if (space < end)
  721                 return (EINVAL);
  722 
  723         if (*slice == NULL) {
  724                 sg = sglist_alloc(count, mflags);
  725                 if (sg == NULL)
  726                         return (ENOMEM);
  727                 *slice = sg;
  728         } else {
  729                 sg = *slice;
  730                 if (sg->sg_maxseg < count)
  731                         return (EFBIG);
  732                 if (sg->sg_nseg != 0)
  733                         return (EINVAL);
  734         }
  735 
  736         /*
  737          * Copy over 'count' segments from 'original' starting at
  738          * 'fseg' to 'sg'.
  739          */
  740         bcopy(original->sg_segs + fseg, sg->sg_segs,
  741             count * sizeof(struct sglist_seg));
  742         sg->sg_nseg = count;
  743 
  744         /* Fixup first and last segments if needed. */
  745         if (foffs != 0) {
  746                 sg->sg_segs[0].ss_paddr += foffs;
  747                 sg->sg_segs[0].ss_len -= foffs;
  748                 CTR2(KTR_DEV, "sglist_slice seg[0]: %08lx:%08lx",
  749                     (long)sg->sg_segs[0].ss_paddr, sg->sg_segs[0].ss_len);
  750         }
  751         if (loffs != 0) {
  752                 sg->sg_segs[count - 1].ss_len -= loffs;
  753                 CTR2(KTR_DEV, "sglist_slice seg[%d]: len %08x", count - 1,
  754                     sg->sg_segs[count - 1].ss_len);
  755         }
  756         return (0);
  757 }

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