The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_physio.c

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    1 /*      $OpenBSD: kern_physio.c,v 1.28 2007/05/26 20:26:51 pedro Exp $  */
    2 /*      $NetBSD: kern_physio.c,v 1.28 1997/05/19 10:43:28 pk Exp $      */
    3 
    4 /*-
    5  * Copyright (c) 1994 Christopher G. Demetriou
    6  * Copyright (c) 1982, 1986, 1990, 1993
    7  *      The Regents of the University of California.  All rights reserved.
    8  * (c) UNIX System Laboratories, Inc.
    9  * All or some portions of this file are derived from material licensed
   10  * to the University of California by American Telephone and Telegraph
   11  * Co. or Unix System Laboratories, Inc. and are reproduced herein with
   12  * the permission of UNIX System Laboratories, Inc.
   13  *
   14  * Redistribution and use in source and binary forms, with or without
   15  * modification, are permitted provided that the following conditions
   16  * are met:
   17  * 1. Redistributions of source code must retain the above copyright
   18  *    notice, this list of conditions and the following disclaimer.
   19  * 2. Redistributions in binary form must reproduce the above copyright
   20  *    notice, this list of conditions and the following disclaimer in the
   21  *    documentation and/or other materials provided with the distribution.
   22  * 3. Neither the name of the University nor the names of its contributors
   23  *    may be used to endorse or promote products derived from this software
   24  *    without specific prior written permission.
   25  *
   26  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   27  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   28  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   29  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   30  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   31  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   32  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   33  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   34  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   35  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   36  * SUCH DAMAGE.
   37  *
   38  *      @(#)kern_physio.c       8.1 (Berkeley) 6/10/93
   39  */
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/buf.h>
   44 #include <sys/conf.h>
   45 #include <sys/proc.h>
   46 #include <sys/pool.h>
   47 
   48 #include <uvm/uvm_extern.h>
   49 
   50 /*
   51  * The routines implemented in this file are described in:
   52  *      Leffler, et al.: The Design and Implementation of the 4.3BSD
   53  *          UNIX Operating System (Addison Welley, 1989)
   54  * on pages 231-233.
   55  *
   56  * The routines "getphysbuf" and "putphysbuf" steal and return a swap
   57  * buffer.  Leffler, et al., says that swap buffers are used to do the
   58  * I/O, so raw I/O requests don't have to be single-threaded.
   59  */
   60 
   61 struct buf *getphysbuf(void);
   62 void putphysbuf(struct buf *bp);
   63 
   64 /*
   65  * Do "physical I/O" on behalf of a user.  "Physical I/O" is I/O directly
   66  * from the raw device to user buffers, and bypasses the buffer cache.
   67  *
   68  * Comments in brackets are from Leffler, et al.'s pseudo-code implementation.
   69  */
   70 int
   71 physio(void (*strategy)(struct buf *), struct buf *bp, dev_t dev, int flags,
   72     void (*minphys)(struct buf *), struct uio *uio)
   73 {
   74         struct iovec *iovp;
   75         struct proc *p = curproc;
   76         int error, done, i, nobuf, s, todo;
   77 
   78         error = 0;
   79         flags &= B_READ | B_WRITE;
   80 
   81         /* Make sure we have a buffer, creating one if necessary. */
   82         if ((nobuf = (bp == NULL)) != 0)
   83                 bp = getphysbuf();
   84 
   85         /* [raise the processor priority level to splbio;] */
   86         s = splbio();
   87 
   88         /* [while the buffer is marked busy] */
   89         while (bp->b_flags & B_BUSY) {
   90                 /* [mark the buffer wanted] */
   91                 bp->b_flags |= B_WANTED;
   92                 /* [wait until the buffer is available] */
   93                 tsleep(bp, PRIBIO+1, "physbuf", 0);
   94         }
   95 
   96         /* Mark it busy, so nobody else will use it. */
   97         bp->b_flags |= B_BUSY;
   98 
   99         /* [lower the priority level] */
  100         splx(s);
  101 
  102         /* [set up the fixed part of the buffer for a transfer] */
  103         bp->b_dev = dev;
  104         bp->b_error = 0;
  105         bp->b_proc = p;
  106         LIST_INIT(&bp->b_dep);
  107 
  108         /*
  109          * [while there are data to transfer and no I/O error]
  110          * Note that I/O errors are handled with a 'goto' at the bottom
  111          * of the 'while' loop.
  112          */
  113         for (i = 0; i < uio->uio_iovcnt; i++) {
  114                 iovp = &uio->uio_iov[i];
  115                 while (iovp->iov_len > 0) {
  116                         /*
  117                          * [mark the buffer busy for physical I/O]
  118                          * (i.e. set B_PHYS (because it's an I/O to user
  119                          * memory), and B_RAW, because B_RAW is to be
  120                          * "Set by physio for raw transfers.", in addition
  121                          * to the "busy" and read/write flag.)
  122                          */
  123                         bp->b_flags = B_BUSY | B_PHYS | B_RAW | flags;
  124 
  125                         /* [set up the buffer for a maximum-sized transfer] */
  126                         bp->b_blkno = btodb(uio->uio_offset);
  127                         bp->b_data = iovp->iov_base;
  128 
  129                         /*
  130                          * Because iov_len is unsigned but b_bcount is signed,
  131                          * an overflow is possible. Therefore bound to MAXPHYS
  132                          * before calling minphys.
  133                          */
  134                         if (iovp->iov_len > MAXPHYS)
  135                                 bp->b_bcount = MAXPHYS;
  136                         else
  137                                 bp->b_bcount = iovp->iov_len;
  138 
  139                         /*
  140                          * [call minphys to bound the transfer size]
  141                          * and remember the amount of data to transfer,
  142                          * for later comparison.
  143                          */
  144                         (*minphys)(bp);
  145                         todo = bp->b_bcount;
  146 #ifdef DIAGNOSTIC
  147                         if (todo < 0)
  148                                 panic("todo < 0; minphys broken");
  149                         if (todo > MAXPHYS)
  150                                 panic("todo > MAXPHYS; minphys broken");
  151 #endif
  152 
  153                         /*
  154                          * [lock the part of the user address space involved
  155                          *    in the transfer]
  156                          * Beware vmapbuf(); it clobbers b_data and
  157                          * saves it in b_saveaddr.  However, vunmapbuf()
  158                          * restores it.
  159                          */
  160                         error = uvm_vslock(p, bp->b_data, todo,
  161                             (flags & B_READ) ?
  162                             VM_PROT_READ | VM_PROT_WRITE : VM_PROT_READ);
  163                         if (error) {
  164                                 bp->b_flags |= B_ERROR;
  165                                 bp->b_error = error;
  166                                 goto after_unlock;
  167                         }
  168                         vmapbuf(bp, todo);
  169 
  170                         /* [call strategy to start the transfer] */
  171                         (*strategy)(bp);
  172 
  173                         /*
  174                          * Note that the raise/wait/lower/get error
  175                          * steps below would be done by biowait(), but
  176                          * we want to unlock the address space before
  177                          * we lower the priority.
  178                          *
  179                          * [raise the priority level to splbio]
  180                          */
  181                         s = splbio();
  182 
  183                         /* [wait for the transfer to complete] */
  184                         while ((bp->b_flags & B_DONE) == 0)
  185                                 tsleep(bp, PRIBIO + 1, "physio", 0);
  186 
  187                         /* Mark it busy again, so nobody else will use it. */
  188                         bp->b_flags |= B_BUSY;
  189 
  190                         /* [lower the priority level] */
  191                         splx(s);
  192 
  193                         /*
  194                          * [unlock the part of the address space previously
  195                          *    locked]
  196                          */
  197                         vunmapbuf(bp, todo);
  198                         uvm_vsunlock(p, bp->b_data, todo);
  199 after_unlock:
  200 
  201                         /* remember error value (save a splbio/splx pair) */
  202                         if (bp->b_flags & B_ERROR)
  203                                 error = (bp->b_error ? bp->b_error : EIO);
  204 
  205                         /*
  206                          * [deduct the transfer size from the total number
  207                          *    of data to transfer]
  208                          */
  209                         done = bp->b_bcount - bp->b_resid;
  210 #ifdef DIAGNOSTIC
  211                         if (done < 0)
  212                                 panic("done < 0; strategy broken");
  213                         if (done > todo)
  214                                 panic("done > todo; strategy broken");
  215 #endif
  216                         iovp->iov_len -= done;
  217                         iovp->iov_base = (caddr_t)iovp->iov_base + done;
  218                         uio->uio_offset += done;
  219                         uio->uio_resid -= done;
  220 
  221                         /*
  222                          * Now, check for an error.
  223                          * Also, handle weird end-of-disk semantics.
  224                          */
  225                         if (error || done < todo)
  226                                 goto done;
  227                 }
  228         }
  229 
  230 done:
  231         /*
  232          * [clean up the state of the buffer]
  233          * Remember if somebody wants it, so we can wake them up below.
  234          * Also, if we had to steal it, give it back.
  235          */
  236         s = splbio();
  237         bp->b_flags &= ~(B_BUSY | B_PHYS | B_RAW);
  238         if (nobuf)
  239                 putphysbuf(bp);
  240         else {
  241                 /*
  242                  * [if another process is waiting for the raw I/O buffer,
  243                  *    wake up processes waiting to do physical I/O]
  244                  */
  245                 if (bp->b_flags & B_WANTED) {
  246                         bp->b_flags &= ~B_WANTED;
  247                         wakeup(bp);
  248                 }
  249         }
  250         splx(s);
  251 
  252         return (error);
  253 }
  254 
  255 /*
  256  * Get a swap buffer structure, for use in physical I/O.
  257  * Mostly taken from /sys/vm/swap_pager.c, except that it no longer
  258  * records buffer list-empty conditions, and sleeps at PRIBIO + 1,
  259  * rather than PSWP + 1 (and on a different wchan).
  260  */
  261 struct buf *
  262 getphysbuf(void)
  263 {
  264         struct buf *bp;
  265         int s;
  266 
  267         s = splbio();
  268         bp = pool_get(&bufpool, PR_WAITOK);
  269         splx(s);
  270 
  271         bzero(bp, sizeof(*bp));
  272 
  273         /* XXXCDC: are the following two lines necessary? */
  274         bp->b_vnbufs.le_next = NOLIST;
  275 
  276         return (bp);
  277 }
  278 
  279 /*
  280  * Get rid of a swap buffer structure which has been used in physical I/O.
  281  * Mostly taken from /sys/vm/swap_pager.c, except that it now uses
  282  * wakeup() rather than the VM-internal thread_wakeup(), and that the caller
  283  * must mask disk interrupts, rather than putphysbuf() itself.
  284  */
  285 void
  286 putphysbuf(struct buf *bp)
  287 {
  288         splassert(IPL_BIO);
  289 
  290         /* XXXCDC: is this necessary? */
  291         if (bp->b_vp)
  292                 brelvp(bp);
  293 
  294 #ifdef DIAGNOSTIC
  295         if (bp->b_flags & B_WANTED)
  296                 panic("putphysbuf: private buf B_WANTED");
  297 #endif
  298         pool_put(&bufpool, bp);
  299 }
  300 
  301 /*
  302  * Leffler, et al., says on p. 231:
  303  * "The minphys() routine is called by physio() to adjust the
  304  * size of each I/O transfer before the latter is passed to
  305  * the strategy routine..."
  306  *
  307  * so, just adjust the buffer's count accounting to MAXPHYS here,
  308  * and return the new count;
  309  */
  310 void
  311 minphys(struct buf *bp)
  312 {
  313 
  314         if (bp->b_bcount > MAXPHYS)
  315                 bp->b_bcount = MAXPHYS;
  316 }

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