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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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