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From: Miklos Szeredi <miklos@szeredi.hu>
To: staubach@redhat.com
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 18:51:54 +0100	[thread overview]
Message-ID: <E1HMSy2-0008F6-00@dorka.pomaz.szeredi.hu> (raw)
In-Reply-To: <45E5BA2E.8060003@redhat.com> (message from Peter Staubach on Wed, 28 Feb 2007 12:21:50 -0500)

> 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.

I disagree.

You don't worry about the timestamp being updated _during_ a large
write() call, even though the file is constantly being modified.

You think of write() as something instantaneous, while you think of
writing to a shared mapping, then doing msync() as something taking a
long time.  In actual fact both of these are basically equivalent
operations, the differences being, that you can easily modify
non-contiguous parts of a file with mmap, while you can't do that with
write.  The disadvantage from mmap comes from the cost of setting up
the page tables and handling the faults.

Think of it this way:

  shared mmap write + msync(MS_ASYNC)  ==  write()
  msync(MS_ASYNC) + fsync()  ==  msync(MS_SYNC)

> 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.

I think this would just be a waste of CPU.

Miklos

  reply	other threads:[~2007-02-28 17:52 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
2007-02-28 17:51         ` Miklos Szeredi [this message]
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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