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From: Peter Staubach <staubach@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [patch 01/22] update ctime and mtime for mmaped write
Date: Wed, 28 Feb 2007 12:21:50 -0500	[thread overview]
Message-ID: <45E5BA2E.8060003@redhat.com> (raw)
In-Reply-To: <E1HMSGE-000886-00@dorka.pomaz.szeredi.hu>

Miklos Szeredi wrote:
>> 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.

While these entry points do not actually modify the file itself,
as was pointed out, they are handy points at which the kernel gains
control and could actually notice that the contents of the file are
no longer the same as they were, ie. modified.

 From the operating system viewpoint, this is where the semantics of
modification to file contents via mmap differs from the semantics of
modification to file contents via write(2).

It is desirable for the file times to be updated as quickly as
possible after the actual modification has occurred.  However, this
can only happen when the kernel has a chance to gain control and
either look or notice that a page has been modified.

---

A better design for all of this would be to update the file times
and mark the inode as needing to be written out when a page fault
is taken for a page which either does not exist or needs to be made
writable and that page is part of an appropriate style mapping.

       ps

  reply	other threads:[~2007-02-28 17:22 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-27 23:14 [patch 00/22] misc VFS/VM patches and fuse writable shared mapping support Miklos Szeredi
2007-02-27 23:14 ` [patch 01/22] update ctime and mtime for mmaped write Miklos Szeredi
2007-02-28 14:16   ` Peter Staubach
2007-02-28 17:06     ` Miklos Szeredi
2007-02-28 17:21       ` Peter Staubach [this message]
2007-02-28 17:51         ` Miklos Szeredi
2007-02-28 20:01           ` Peter Staubach
2007-02-28 20:35             ` Miklos Szeredi
2007-02-28 20:58               ` Miklos Szeredi
2007-02-28 21:09                 ` Peter Staubach
2007-03-01  7:25                   ` Miklos Szeredi
2007-02-27 23:14 ` [patch 02/22] fix quadratic behavior of shrink_dcache_parent() Miklos Szeredi
2007-02-27 23:14 ` [patch 03/22] fix deadlock in balance_dirty_pages Miklos Szeredi
2007-02-27 23:14 ` [patch 04/22] fix deadlock in throttle_vm_writeout Miklos Szeredi
2007-02-27 23:14 ` [patch 05/22] balance dirty pages from loop device Miklos Szeredi
2007-02-27 23:14 ` [patch 06/22] consolidate generic_writepages and mpage_writepages Miklos Szeredi
2007-02-27 23:14 ` [patch 07/22] add filesystem subtype support Miklos Szeredi
2007-02-27 23:14 ` [patch 08/22] fuse: update backing_dev_info congestion state Miklos Szeredi
2007-02-27 23:14 ` [patch 09/22] fuse: fix reserved request wake up Miklos Szeredi
2007-02-27 23:14 ` [patch 10/22] fuse: add reference counting to fuse_file Miklos Szeredi
2007-02-27 23:14 ` [patch 11/22] fuse: add truncation semaphore Miklos Szeredi
2007-02-27 23:14 ` [patch 12/22] fuse: fix page invalidation Miklos Szeredi
2007-02-27 23:14 ` [patch 13/22] fuse: add list of writable files to fuse_inode Miklos Szeredi
2007-02-27 23:14 ` [patch 14/22] fuse: add helper for asynchronous writes Miklos Szeredi
2007-02-27 23:14 ` [patch 15/22] add non-owner variant of down_read_trylock() Miklos Szeredi
2007-02-27 23:14 ` [patch 16/22] fuse: add fuse_writepage() function Miklos Szeredi
2007-02-27 23:14 ` [patch 17/22] fuse: writable shared mmap support Miklos Szeredi
2007-02-27 23:15 ` [patch 18/22] fuse: add fuse_writepages() function Miklos Szeredi
2007-02-27 23:15 ` [patch 19/22] export sync_sb() to modules Miklos Szeredi
2007-02-27 23:15 ` [patch 20/22] fuse: make dirty stats available Miklos Szeredi
2007-02-27 23:15 ` [patch 21/22] fuse: limit dirty pages Miklos Szeredi
2007-02-27 23:15 ` [patch 22/22] fuse: allow big write requests Miklos Szeredi

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