Re: [PATCH -v7 2/2] Update ctime and mtime for memory-mapped files

["Jesper Juhl" <jesper.juhl@gmail.com>]

Some very pedantic nitpicking below;

On 22/01/2008, Anton Salikhmetov <salikhmetov@gmail.com> wrote:
> http://bugzilla.kernel.org/show_bug.cgi?id=2645#c40
>
> Update file times at write references to memory-mapped files.
> Force 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  |   57 ++++++++++++++++++++++++++++++++++++++++++++-------------
>  2 files changed, 50 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6dd1cd8..4b0144b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1670,6 +1670,9 @@ 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
> @@ -2343,6 +2346,9 @@ 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..394130d 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -5,6 +5,7 @@
>   * Copyright (C) 2008 Anton Salikhmetov <salikhmetov@gmail.com>
>   */
>
> +#include <asm/tlbflush.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/mm.h>
> @@ -13,11 +14,37 @@
>  #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) {

I know it's not the common "Linux Kernel way", but 'addr' could be
made to have just 'for' scope here according to C99;

       for (unsigned long 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_t entry = ptep_clear_flush(vma, addr, pte);
> +
> +                       entry = pte_wrprotect(entry);
> +                       set_pte_at(vma->vm_mm, addr, pte, entry);
> +               }
> +               pte_unmap_unlock(pte, ptl);
> +       }
> +}
> +
> +/*
>   * 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.

I think keeping some version of the "up to ..." comments makes sense.
It documents that we previously had different behaviour.

> + * 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 +104,20 @@ 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) {

"else if" ??

> +                               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;

-- 
Jesper Juhl <jesper.juhl@gmail.com>
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