FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/include/os/linux/spl/sys/vmem.h

     /*
      *  Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
      *  Copyright (C) 2007 The Regents of the University of California.
      *  Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
      *  Written by Brian Behlendorf <behlendorf1@llnl.gov>.
      *  UCRL-CODE-235197
      *
      *  This file is part of the SPL, Solaris Porting Layer.
      *
     *  The SPL is free software; you can redistribute it and/or modify it
     *  under the terms of the GNU General Public License as published by the
     *  Free Software Foundation; either version 2 of the License, or (at your
     *  option) any later version.
     *
     *  The SPL is distributed in the hope that it will be useful, but WITHOUT
     *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
     *  for more details.
     *
     *  You should have received a copy of the GNU General Public License along
     *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
     */
    
    #ifndef _SPL_VMEM_H
    #define _SPL_VMEM_H
    
    #include <sys/kmem.h>
    #include <linux/sched.h>
    #include <linux/vmalloc.h>
    
    typedef struct vmem { } vmem_t;
    
    /*
     * Memory allocation interfaces
     */
    #define VMEM_ALLOC      0x01
    #define VMEM_FREE       0x02
    
    #ifndef VMALLOC_TOTAL
    #define VMALLOC_TOTAL   (VMALLOC_END - VMALLOC_START)
    #endif
    
    /*
     * vmem_* is an interface to a low level arena-based memory allocator on
     * Illumos that is used to allocate virtual address space. The kmem SLAB
     * allocator allocates slabs from it. Then the generic allocation functions
     * kmem_{alloc,zalloc,free}() are layered on top of SLAB allocators.
     *
     * On Linux, the primary means of doing allocations is via kmalloc(), which
     * is similarly layered on top of something called the buddy allocator. The
     * buddy allocator is not available to kernel modules, it uses physical
     * memory addresses rather than virtual memory addresses and is prone to
     * fragmentation.
     *
     * Linux sets aside a relatively small address space for in-kernel virtual
     * memory from which allocations can be done using vmalloc().  It might seem
     * like a good idea to use vmalloc() to implement something similar to
     * Illumos' allocator. However, this has the following problems:
     *
     * 1. Page directory table allocations are hard coded to use GFP_KERNEL.
     *    Consequently, any KM_PUSHPAGE or KM_NOSLEEP allocations done using
     *    vmalloc() will not have proper semantics.
     *
     * 2. Address space exhaustion is a real issue on 32-bit platforms where
     *    only a few 100MB are available. The kernel will handle it by spinning
     *    when it runs out of address space.
     *
     * 3. All vmalloc() allocations and frees are protected by a single global
     *    lock which serializes all allocations.
     *
     * 4. Accessing /proc/meminfo and /proc/vmallocinfo will iterate the entire
     *    list. The former will sum the allocations while the latter will print
     *    them to user space in a way that user space can keep the lock held
     *    indefinitely.  When the total number of mapped allocations is large
     *    (several 100,000) a large amount of time will be spent waiting on locks.
     *
     * 5. Linux has a wait_on_bit() locking primitive that assumes physical
     *    memory is used, it simply does not work on virtual memory.  Certain
     *    Linux structures (e.g. the superblock) use them and might be embedded
     *    into a structure from Illumos.  This makes using Linux virtual memory
     *    unsafe in certain situations.
     *
     * It follows that we cannot obtain identical semantics to those on Illumos.
     * Consequently, we implement the kmem_{alloc,zalloc,free}() functions in
     * such a way that they can be used as drop-in replacements for small vmem_*
     * allocations (8MB in size or smaller) and map vmem_{alloc,zalloc,free}()
     * to them.
     */
    
    #define vmem_alloc(sz, fl)      spl_vmem_alloc((sz), (fl), __func__, __LINE__)
    #define vmem_zalloc(sz, fl)     spl_vmem_zalloc((sz), (fl), __func__, __LINE__)
    #define vmem_free(ptr, sz)      spl_vmem_free((ptr), (sz))
    
    extern void *spl_vmem_alloc(size_t sz, int fl, const char *func, int line);
    extern void *spl_vmem_zalloc(size_t sz, int fl, const char *func, int line);
    extern void spl_vmem_free(const void *ptr, size_t sz);
    
    int spl_vmem_init(void);
    void spl_vmem_fini(void);
   
   #endif  /* _SPL_VMEM_H */

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