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    SubjectRe: [PATCH] update ctime and mtime for mmaped write
    Miklos Szeredi wrote:
    >> On Wed, 21 Feb 2007 18:51:52 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:
    >>
    >>
    >>> This patch makes writing to shared memory mappings update st_ctime and
    >>> st_mtime as defined by SUSv3:
    >>>
    >>> The st_ctime and st_mtime fields of a file that is mapped with
    >>> MAP_SHARED and PROT_WRITE shall be marked for update at some point
    >>> in the interval between a write reference to the mapped region and
    >>> the next call to msync() with MS_ASYNC or MS_SYNC for that portion
    >>> of the file by any process. If there is no such call and if the
    >>> underlying file is modified as a result of a write reference, then
    >>> these fields shall be marked for update at some time after the
    >>> write reference.
    >>>
    >>> A new address_space flag is introduced: AS_CMTIME. This is set each
    >>> time a page is dirtied through a userspace memory mapping. This
    >>> includes write accesses via get_user_pages().
    >>>
    >>> Note, the flag is set unconditionally, even if the page is already
    >>> dirty. This is important, because the page might have been dirtied
    >>> earlier by a non-mmap write.
    >>>
    >>> This flag is checked in msync() and __fput(), and if set, the file
    >>> times are updated and the flag is cleared
    >>>
    >>> The flag is also cleared, if the time update is triggered by a normal
    >>> write. This is not mandated by the standard, but seems to be a sane
    >>> thing to do.
    >>>
    >> Why is the flag checked in __fput()?
    >>
    >
    > It's because of this bit in the standard:
    >
    > If there is no such call and if the underlying file is modified
    > as a result of a write reference, then these fields shall be
    > marked for update at some time after the write reference.
    >
    > It could be done in munmap/mremap, but it seemed more difficult to
    > track down all the places where the vma is removed. But yes, that may
    > be a nicer solution.

    It seems to me that, with this support, a file, which is mmap'd,
    modified, but never msync'd or munmap'd, will never get its mtime
    updated. Or did I miss that?

    I also don't see how an mmap'd block device will get its mtime
    updated either.

    Thanx...

    ps
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