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![Kirill A. Shutemov](/assets/img/avatar_default.png)
The remap_file_pages() system call is used to create a nonlinear mapping, that is, a mapping in which the pages of the file are mapped into a nonsequential order in memory. The advantage of using remap_file_pages() over using repeated calls to mmap(2) is that the former approach does not require the kernel to create additional VMA (Virtual Memory Area) data structures. Supporting of nonlinear mapping requires significant amount of non-trivial code in kernel virtual memory subsystem including hot paths. Also to get nonlinear mapping work kernel need a way to distinguish normal page table entries from entries with file offset (pte_file). Kernel reserves flag in PTE for this purpose. PTE flags are scarce resource especially on some CPU architectures. It would be nice to free up the flag for other usage. Fortunately, there are not many users of remap_file_pages() in the wild. It's only known that one enterprise RDBMS implementation uses the syscall on 32-bit systems to map files bigger than can linearly fit into 32-bit virtual address space. This use-case is not critical anymore since 64-bit systems are widely available. The plan is to deprecate the syscall and replace it with an emulation. The emulation will create new VMAs instead of nonlinear mappings. It's going to work slower for rare users of remap_file_pages() but ABI is preserved. One side effect of emulation (apart from performance) is that user can hit vm.max_map_count limit more easily due to additional VMAs. See comment for DEFAULT_MAX_MAP_COUNT for more details on the limit. [akpm@linux-foundation.org: fix spello] Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Dave Jones <davej@redhat.com> Cc: Armin Rigo <arigo@tunes.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
283 lines
7.3 KiB
C
283 lines
7.3 KiB
C
/*
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* linux/mm/fremap.c
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*
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* Explicit pagetable population and nonlinear (random) mappings support.
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*
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* started by Ingo Molnar, Copyright (C) 2002, 2003
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*/
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#include <linux/export.h>
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#include <linux/backing-dev.h>
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#include <linux/mm.h>
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#include <linux/swap.h>
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#include <linux/file.h>
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#include <linux/mman.h>
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#include <linux/pagemap.h>
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#include <linux/swapops.h>
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#include <linux/rmap.h>
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#include <linux/syscalls.h>
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#include <linux/mmu_notifier.h>
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#include <asm/mmu_context.h>
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#include <asm/cacheflush.h>
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#include <asm/tlbflush.h>
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#include "internal.h"
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static int mm_counter(struct page *page)
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{
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return PageAnon(page) ? MM_ANONPAGES : MM_FILEPAGES;
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}
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static void zap_pte(struct mm_struct *mm, struct vm_area_struct *vma,
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unsigned long addr, pte_t *ptep)
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{
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pte_t pte = *ptep;
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struct page *page;
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swp_entry_t entry;
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if (pte_present(pte)) {
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flush_cache_page(vma, addr, pte_pfn(pte));
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pte = ptep_clear_flush(vma, addr, ptep);
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page = vm_normal_page(vma, addr, pte);
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if (page) {
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if (pte_dirty(pte))
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set_page_dirty(page);
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update_hiwater_rss(mm);
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dec_mm_counter(mm, mm_counter(page));
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page_remove_rmap(page);
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page_cache_release(page);
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}
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} else { /* zap_pte() is not called when pte_none() */
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if (!pte_file(pte)) {
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update_hiwater_rss(mm);
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entry = pte_to_swp_entry(pte);
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if (non_swap_entry(entry)) {
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if (is_migration_entry(entry)) {
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page = migration_entry_to_page(entry);
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dec_mm_counter(mm, mm_counter(page));
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}
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} else {
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free_swap_and_cache(entry);
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dec_mm_counter(mm, MM_SWAPENTS);
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}
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}
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pte_clear_not_present_full(mm, addr, ptep, 0);
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}
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}
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/*
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* Install a file pte to a given virtual memory address, release any
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* previously existing mapping.
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*/
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static int install_file_pte(struct mm_struct *mm, struct vm_area_struct *vma,
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unsigned long addr, unsigned long pgoff, pgprot_t prot)
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{
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int err = -ENOMEM;
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pte_t *pte, ptfile;
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spinlock_t *ptl;
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pte = get_locked_pte(mm, addr, &ptl);
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if (!pte)
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goto out;
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ptfile = pgoff_to_pte(pgoff);
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if (!pte_none(*pte))
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zap_pte(mm, vma, addr, pte);
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set_pte_at(mm, addr, pte, pte_file_mksoft_dirty(ptfile));
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/*
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* We don't need to run update_mmu_cache() here because the "file pte"
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* being installed by install_file_pte() is not a real pte - it's a
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* non-present entry (like a swap entry), noting what file offset should
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* be mapped there when there's a fault (in a non-linear vma where
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* that's not obvious).
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*/
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pte_unmap_unlock(pte, ptl);
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err = 0;
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out:
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return err;
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}
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int generic_file_remap_pages(struct vm_area_struct *vma, unsigned long addr,
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unsigned long size, pgoff_t pgoff)
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{
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struct mm_struct *mm = vma->vm_mm;
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int err;
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do {
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err = install_file_pte(mm, vma, addr, pgoff, vma->vm_page_prot);
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if (err)
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return err;
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size -= PAGE_SIZE;
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addr += PAGE_SIZE;
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pgoff++;
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} while (size);
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return 0;
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}
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EXPORT_SYMBOL(generic_file_remap_pages);
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/**
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* sys_remap_file_pages - remap arbitrary pages of an existing VM_SHARED vma
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* @start: start of the remapped virtual memory range
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* @size: size of the remapped virtual memory range
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* @prot: new protection bits of the range (see NOTE)
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* @pgoff: to-be-mapped page of the backing store file
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* @flags: 0 or MAP_NONBLOCKED - the later will cause no IO.
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*
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* sys_remap_file_pages remaps arbitrary pages of an existing VM_SHARED vma
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* (shared backing store file).
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*
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* This syscall works purely via pagetables, so it's the most efficient
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* way to map the same (large) file into a given virtual window. Unlike
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* mmap()/mremap() it does not create any new vmas. The new mappings are
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* also safe across swapout.
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*
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* NOTE: the @prot parameter right now is ignored (but must be zero),
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* and the vma's default protection is used. Arbitrary protections
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* might be implemented in the future.
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*/
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SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
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unsigned long, prot, unsigned long, pgoff, unsigned long, flags)
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{
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struct mm_struct *mm = current->mm;
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struct address_space *mapping;
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struct vm_area_struct *vma;
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int err = -EINVAL;
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int has_write_lock = 0;
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vm_flags_t vm_flags = 0;
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pr_warn_once("%s (%d) uses deprecated remap_file_pages() syscall. "
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"See Documentation/vm/remap_file_pages.txt.\n",
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current->comm, current->pid);
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if (prot)
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return err;
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/*
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* Sanitize the syscall parameters:
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*/
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start = start & PAGE_MASK;
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size = size & PAGE_MASK;
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/* Does the address range wrap, or is the span zero-sized? */
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if (start + size <= start)
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return err;
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/* Does pgoff wrap? */
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if (pgoff + (size >> PAGE_SHIFT) < pgoff)
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return err;
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/* Can we represent this offset inside this architecture's pte's? */
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#if PTE_FILE_MAX_BITS < BITS_PER_LONG
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if (pgoff + (size >> PAGE_SHIFT) >= (1UL << PTE_FILE_MAX_BITS))
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return err;
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#endif
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/* We need down_write() to change vma->vm_flags. */
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down_read(&mm->mmap_sem);
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retry:
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vma = find_vma(mm, start);
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/*
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* Make sure the vma is shared, that it supports prefaulting,
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* and that the remapped range is valid and fully within
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* the single existing vma.
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*/
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if (!vma || !(vma->vm_flags & VM_SHARED))
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goto out;
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if (!vma->vm_ops || !vma->vm_ops->remap_pages)
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goto out;
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if (start < vma->vm_start || start + size > vma->vm_end)
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goto out;
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/* Must set VM_NONLINEAR before any pages are populated. */
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if (!(vma->vm_flags & VM_NONLINEAR)) {
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/*
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* vm_private_data is used as a swapout cursor
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* in a VM_NONLINEAR vma.
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*/
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if (vma->vm_private_data)
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goto out;
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/* Don't need a nonlinear mapping, exit success */
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if (pgoff == linear_page_index(vma, start)) {
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err = 0;
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goto out;
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}
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if (!has_write_lock) {
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get_write_lock:
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up_read(&mm->mmap_sem);
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down_write(&mm->mmap_sem);
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has_write_lock = 1;
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goto retry;
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}
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mapping = vma->vm_file->f_mapping;
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/*
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* page_mkclean doesn't work on nonlinear vmas, so if
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* dirty pages need to be accounted, emulate with linear
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* vmas.
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*/
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if (mapping_cap_account_dirty(mapping)) {
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unsigned long addr;
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struct file *file = get_file(vma->vm_file);
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/* mmap_region may free vma; grab the info now */
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vm_flags = vma->vm_flags;
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addr = mmap_region(file, start, size, vm_flags, pgoff);
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fput(file);
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if (IS_ERR_VALUE(addr)) {
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err = addr;
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} else {
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BUG_ON(addr != start);
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err = 0;
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}
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goto out_freed;
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}
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mutex_lock(&mapping->i_mmap_mutex);
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flush_dcache_mmap_lock(mapping);
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vma->vm_flags |= VM_NONLINEAR;
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vma_interval_tree_remove(vma, &mapping->i_mmap);
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vma_nonlinear_insert(vma, &mapping->i_mmap_nonlinear);
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flush_dcache_mmap_unlock(mapping);
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mutex_unlock(&mapping->i_mmap_mutex);
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}
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if (vma->vm_flags & VM_LOCKED) {
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/*
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* drop PG_Mlocked flag for over-mapped range
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*/
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if (!has_write_lock)
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goto get_write_lock;
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vm_flags = vma->vm_flags;
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munlock_vma_pages_range(vma, start, start + size);
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vma->vm_flags = vm_flags;
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}
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mmu_notifier_invalidate_range_start(mm, start, start + size);
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err = vma->vm_ops->remap_pages(vma, start, size, pgoff);
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mmu_notifier_invalidate_range_end(mm, start, start + size);
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/*
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* We can't clear VM_NONLINEAR because we'd have to do
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* it after ->populate completes, and that would prevent
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* downgrading the lock. (Locks can't be upgraded).
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*/
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out:
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if (vma)
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vm_flags = vma->vm_flags;
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out_freed:
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if (likely(!has_write_lock))
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up_read(&mm->mmap_sem);
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else
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up_write(&mm->mmap_sem);
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if (!err && ((vm_flags & VM_LOCKED) || !(flags & MAP_NONBLOCK)))
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mm_populate(start, size);
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return err;
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}
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