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 andWhy is the flag checked in __fput()?
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
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.
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.