Re: [PATCH] update ctime and mtime for mmaped write

From: Andrew Morton
Date: Wed Feb 21 2007 - 23:26:49 EST

On Wed, 21 Feb 2007 18:51:52 +0100 Miklos Szeredi <miklos@xxxxxxxxxx> 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()?
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