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SubjectRe: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files
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> Updating file times at write references to memory-mapped files and
> forcing file times update at the next write reference after
> calling the msync() system call with the MS_ASYNC flag.
>
> Signed-off-by: Anton Salikhmetov <salikhmetov@gmail.com>
> ---
> mm/memory.c | 6 ++++++
> mm/msync.c | 52 +++++++++++++++++++++++++++++++++++++++-------------
> 2 files changed, 45 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 4bf0b6d..13d5bbf 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1668,6 +1668,9 @@ gotten:
> unlock:
> pte_unmap_unlock(page_table, ptl);
> if (dirty_page) {
> + if (vma->vm_file)
> + file_update_time(vma->vm_file);
> +
> /*
> * Yes, Virginia, this is actually required to prevent a race
> * with clear_page_dirty_for_io() from clearing the page dirty
> @@ -2341,6 +2344,9 @@ out_unlocked:
> if (anon)
> page_cache_release(vmf.page);
> else if (dirty_page) {
> + if (vma->vm_file)
> + file_update_time(vma->vm_file);
> +
> set_page_dirty_balance(dirty_page, page_mkwrite);
> put_page(dirty_page);
> }
> diff --git a/mm/msync.c b/mm/msync.c
> index a4de868..a49af28 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -13,11 +13,33 @@
> #include <linux/syscalls.h>
>
> /*
> + * Scan the PTEs for pages belonging to the VMA and mark them read-only.
> + * It will force a pagefault on the next write access.
> + */
> +static void vma_wrprotect(struct vm_area_struct *vma)
> +{
> + unsigned long addr;
> +
> + for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
> + spinlock_t *ptl;
> + pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> + pud_t *pud = pud_offset(pgd, addr);
> + pmd_t *pmd = pmd_offset(pud, addr);
> + pte_t *pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> +
> + if (pte_dirty(*pte) && pte_write(*pte))
> + *pte = pte_wrprotect(*pte);
> + pte_unmap_unlock(pte, ptl);
> + }
> +}

What about ram based filesystems? They don't start out with read-only
pte's, so I think they don't want them read-protected now either.
Unless this is essential for correct mtime/ctime accounting on these
filesystems (I don't think it really is). But then the mapping should
start out read-only as well, otherwise the time update will only work
after an msync(MS_ASYNC).

> +
> +/*
> * MS_SYNC syncs the entire file - including mappings.
> *
> - * MS_ASYNC does not start I/O (it used to, up to 2.5.67).
> - * Nor does it mark the relevant pages dirty (it used to up to 2.6.17).
> - * Now it doesn't do anything, since dirty pages are properly tracked.
> + * MS_ASYNC does not start I/O. Instead, it marks the relevant pages
> + * read-only by calling vma_wrprotect(). This is needed to catch the next
> + * write reference to the mapped region and update the file times
> + * accordingly.
> *
> * The application may now run fsync() to write out the dirty pages and
> * wait on the writeout and check the result. Or the application may run
> @@ -77,16 +99,20 @@ asmlinkage long sys_msync(unsigned long start, size_t len, int flags)
> error = 0;
> start = vma->vm_end;
> file = vma->vm_file;
> - if (file && (vma->vm_flags & VM_SHARED) && (flags & MS_SYNC)) {
> - get_file(file);
> - up_read(&mm->mmap_sem);
> - error = do_fsync(file, 0);
> - fput(file);
> - if (error || start >= end)
> - goto out;
> - down_read(&mm->mmap_sem);
> - vma = find_vma(mm, start);
> - continue;
> + if (file && (vma->vm_flags & VM_SHARED)) {
> + if (flags & MS_ASYNC)
> + vma_wrprotect(vma);
> + if (flags & MS_SYNC) {
> + get_file(file);
> + up_read(&mm->mmap_sem);
> + error = do_fsync(file, 0);
> + fput(file);
> + if (error || start >= end)
> + goto out;
> + down_read(&mm->mmap_sem);
> + vma = find_vma(mm, start);
> + continue;
> + }
> }
>
> vma = vma->vm_next;
> --
> 1.4.4.4
>
>


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