linux-kernel - Re: [patch 01/22] update ctime and mtime for mmaped write

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Message-Id: <E1HMSGE-000886-00@dorka.pomaz.szeredi.hu>
Date:	Wed, 28 Feb 2007 18:06:38 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	staubach@...hat.com
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [patch 01/22] update ctime and mtime for mmaped write

> These change still have the undesirable property that although the
> modified pages may be flushed to stable storage, the metadata on
> the file will not be updated until the application takes positive
> action.  This is permissible given the current wording in the
> specifications, but it would be much more desirable if sync(2),
> fsync(P), or the inode being written out due to normal system
> activity would also cause the metadata to be updated.
> 
> Perhaps the setting of the flag could be checked in some places
> like __sync_single_inode() and do_fsync()?

I don't see the point in updating the timestamp from these functions.

The file isn't _modified_ by sync() or fsync().  Just as it's not
modified by stat().

sync() and fsync() do cache->disk, while the file itself stays the
same.

OTOH msync(MS_ASYNC) does memory->file, which is a conceptually file
modifying operation.  OK, msync(MS_ASYNC) is actually a no-op on
2.6.18+, but that's purely an implementation detail and no application
should be relying on it.

Before 2.6.18 sync() or fsync() acually didn't flush data written
through a shared mapping to disk, only msync(MS_SYNC), because the
dirty state was only available in the page tables, not in the page or
the inode.

Miklos
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