From: David Howells The attached patch adds a new VMA operation to notify a filesystem or other driver about the MMU generating a fault because userspace attempted to write to a page mapped through a read-only PTE. This facility permits the filesystem or driver to: (*) Implement storage allocation/reservation on attempted write, and so to deal with problems such as ENOSPC more gracefully (perhaps by generating SIGBUS). (*) Delay making the page writable until the contents have been written to a backing cache. This is useful for NFS/AFS when using FS-Cache/CacheFS. It permits the filesystem to have some guarantee about the state of the cache. Signed-Off-By: David Howells Cc: Hugh Dickins Cc: Anton Altaparmakov Signed-off-by: Andrew Morton --- include/linux/mm.h | 4 ++ mm/memory.c | 95 +++++++++++++++++++++++++++++++++++++++-------------- mm/mmap.c | 9 +++-- mm/mprotect.c | 8 +++- 4 files changed, 88 insertions(+), 28 deletions(-) diff -puN include/linux/mm.h~add-notification-of-page-becoming-writable-to-vma-ops include/linux/mm.h --- devel/include/linux/mm.h~add-notification-of-page-becoming-writable-to-vma-ops 2005-11-05 02:23:10.000000000 -0800 +++ devel-akpm/include/linux/mm.h 2005-11-05 02:23:10.000000000 -0800 @@ -196,6 +196,10 @@ struct vm_operations_struct { void (*close)(struct vm_area_struct * area); struct page * (*nopage)(struct vm_area_struct * area, unsigned long address, int *type); int (*populate)(struct vm_area_struct * area, unsigned long address, unsigned long len, pgprot_t prot, unsigned long pgoff, int nonblock); + + /* notification that a previously read-only page is about to become + * writable, if an error is returned it will cause a SIGBUS */ + int (*page_mkwrite)(struct vm_area_struct *vma, struct page *page); #ifdef CONFIG_NUMA int (*set_policy)(struct vm_area_struct *vma, struct mempolicy *new); struct mempolicy *(*get_policy)(struct vm_area_struct *vma, diff -puN mm/memory.c~add-notification-of-page-becoming-writable-to-vma-ops mm/memory.c --- devel/mm/memory.c~add-notification-of-page-becoming-writable-to-vma-ops 2005-11-05 02:23:10.000000000 -0800 +++ devel-akpm/mm/memory.c 2005-11-05 02:23:10.000000000 -0800 @@ -1274,7 +1274,7 @@ static int do_wp_page(struct mm_struct * struct page *old_page, *new_page; unsigned long pfn = pte_pfn(orig_pte); pte_t entry; - int ret = VM_FAULT_MINOR; + int reuse, ret = VM_FAULT_MINOR; BUG_ON(vma->vm_flags & VM_RESERVED); @@ -1288,19 +1288,49 @@ static int do_wp_page(struct mm_struct * } old_page = pfn_to_page(pfn); - if (PageAnon(old_page) && !TestSetPageLocked(old_page)) { - int reuse = can_share_swap_page(old_page); - unlock_page(old_page); - if (reuse) { - flush_cache_page(vma, address, pfn); - entry = pte_mkyoung(orig_pte); - entry = maybe_mkwrite(pte_mkdirty(entry), vma); - ptep_set_access_flags(vma, address, page_table, entry, 1); - update_mmu_cache(vma, address, entry); - lazy_mmu_prot_update(entry); - ret |= VM_FAULT_WRITE; - goto unlock; + if (unlikely(vma->vm_flags & VM_SHARED)) { + if (vma->vm_ops && vma->vm_ops->page_mkwrite) { + /* + * Notify the page owner without the lock held, + * so they can sleep if they want to. + */ + page_cache_get(old_page); + pte_unmap_unlock(page_table, ptl); + + if (vma->vm_ops->page_mkwrite(vma, old_page) < 0) + goto unwritable_page; + + page_cache_release(old_page); + + /* + * Since we dropped the lock we need to revalidate + * the PTE as someone else may have changed it. If + * they did, we just return, as we can count on the + * MMU to tell us if they didn't also make it writable. + */ + page_table = pte_offset_map_lock(mm, pmd, address, + &ptl); + if (!pte_same(*page_table, orig_pte)) + goto unlock; } + + reuse = 1; + } else if (PageAnon(old_page) && !TestSetPageLocked(old_page)) { + reuse = can_share_swap_page(old_page); + unlock_page(old_page); + } else { + reuse = 0; + } + + if (reuse) { + flush_cache_page(vma, address, pfn); + entry = pte_mkyoung(orig_pte); + entry = maybe_mkwrite(pte_mkdirty(entry), vma); + ptep_set_access_flags(vma, address, page_table, entry, 1); + update_mmu_cache(vma, address, entry); + lazy_mmu_prot_update(entry); + ret |= VM_FAULT_WRITE; + goto unlock; } /* @@ -1353,6 +1383,10 @@ unlock: oom: page_cache_release(old_page); return VM_FAULT_OOM; + +unwritable_page: + page_cache_release(old_page); + return VM_FAULT_SIGBUS; } /* @@ -1874,18 +1908,31 @@ retry: /* * Should we do an early C-O-W break? */ - if (write_access && !(vma->vm_flags & VM_SHARED)) { - struct page *page; + if (write_access) { + if (!(vma->vm_flags & VM_SHARED)) { + struct page *page; - if (unlikely(anon_vma_prepare(vma))) - goto oom; - page = alloc_page_vma(GFP_HIGHUSER, vma, address); - if (!page) - goto oom; - copy_user_highpage(page, new_page, address); - page_cache_release(new_page); - new_page = page; - anon = 1; + if (unlikely(anon_vma_prepare(vma))) + goto oom; + page = alloc_page_vma(GFP_HIGHUSER, vma, address); + if (!page) + goto oom; + copy_user_highpage(page, new_page, address); + page_cache_release(new_page); + new_page = page; + anon = 1; + + } else { + /* if the page will be shareable, see if the backing + * address space wants to know that the page is about + * to become writable */ + if (vma->vm_ops->page_mkwrite && + vma->vm_ops->page_mkwrite(vma, new_page) < 0 + ) { + page_cache_release(new_page); + return VM_FAULT_SIGBUS; + } + } } page_table = pte_offset_map_lock(mm, pmd, address, &ptl); diff -puN mm/mmap.c~add-notification-of-page-becoming-writable-to-vma-ops mm/mmap.c --- devel/mm/mmap.c~add-notification-of-page-becoming-writable-to-vma-ops 2005-11-05 02:23:10.000000000 -0800 +++ devel-akpm/mm/mmap.c 2005-11-05 02:23:10.000000000 -0800 @@ -1062,7 +1062,8 @@ munmap_back: vma->vm_start = addr; vma->vm_end = addr + len; vma->vm_flags = vm_flags; - vma->vm_page_prot = protection_map[vm_flags & 0x0f]; + vma->vm_page_prot = protection_map[vm_flags & + (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]; vma->vm_pgoff = pgoff; if (file) { @@ -1096,6 +1097,9 @@ munmap_back: if (error) goto free_vma; } + if (vma->vm_ops && vma->vm_ops->page_mkwrite) + vma->vm_page_prot = protection_map[vm_flags & + (VM_READ|VM_WRITE|VM_EXEC)]; /* We set VM_ACCOUNT in a shared mapping's vm_flags, to inform * shmem_zero_setup (perhaps called through /dev/zero's ->mmap) @@ -1930,7 +1934,8 @@ unsigned long do_brk(unsigned long addr, vma->vm_end = addr + len; vma->vm_pgoff = pgoff; vma->vm_flags = flags; - vma->vm_page_prot = protection_map[flags & 0x0f]; + vma->vm_page_prot = protection_map[flags & + (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]; vma_link(mm, vma, prev, rb_link, rb_parent); out: mm->total_vm += len >> PAGE_SHIFT; diff -puN mm/mprotect.c~add-notification-of-page-becoming-writable-to-vma-ops mm/mprotect.c --- devel/mm/mprotect.c~add-notification-of-page-becoming-writable-to-vma-ops 2005-11-05 02:23:10.000000000 -0800 +++ devel-akpm/mm/mprotect.c 2005-11-05 02:23:10.000000000 -0800 @@ -106,6 +106,7 @@ mprotect_fixup(struct vm_area_struct *vm unsigned long oldflags = vma->vm_flags; long nrpages = (end - start) >> PAGE_SHIFT; unsigned long charged = 0; + unsigned int mask; pgprot_t newprot; pgoff_t pgoff; int error; @@ -140,8 +141,6 @@ mprotect_fixup(struct vm_area_struct *vm } } - newprot = protection_map[newflags & 0xf]; - /* * First try to merge with previous and/or next vma. */ @@ -168,6 +167,11 @@ mprotect_fixup(struct vm_area_struct *vm } success: + mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED; + if (vma->vm_ops && vma->vm_ops->page_mkwrite) + mask &= ~VM_SHARED; + newprot = protection_map[newflags & mask]; + /* * vm_flags and vm_page_prot are protected by the mmap_sem * held in write mode. _