Re: [PATCH 00/11 v2] Push file_update_time() into .page_mkwrite

[Andy Lutomirski <luto@amacapital.net>]

On 03/01/2012 03:41 AM, Jan Kara wrote:
>   Hello,
> 
>   to provide reliable support for filesystem freezing, filesystems need to have
> complete control over when metadata is changed. In particular,
> file_update_time() calls from page fault code make it impossible for
> filesystems to prevent inodes from being dirtied while the filesystem is
> frozen.
> 
> To fix the issue, this patch set changes page fault code to call
> file_update_time() only when ->page_mkwrite() callback is not provided. If the
> callback is provided, it is the responsibility of the filesystem to perform
> update of i_mtime / i_ctime if needed. We also push file_update_time() call
> to all existing ->page_mkwrite() implementations if the time update does not
> obviously happen by other means. If you know your filesystem does not need
> update of modification times in ->page_mkwrite() handler, please speak up and
> I'll drop the patch for your filesystem.
> 
> As a side note, an alternative would be to remove call of file_update_time()
> from page fault code altogether and require all filesystems needing it to do
> that in their ->page_mkwrite() implementation. That is certainly possible
> although maybe slightly inefficient and would require auditting 100+
> vm_operations_structs *shiver*.



IMO updating file times should happen when changes get written out, not
when a page is made writable, for two reasons:

1. Correctness.  With the current approach, it's very easy for files to
be changed after the last mtime update -- any changes between mkwrite
and actual writeback won't affect mtime.

2. Performance.  I have an application (presumably guessable from my
email address) for which blocking in page_mkwrite is an absolute
show-stopper.  (In fact it's so bad that we reverted back to running on
Windows until I hacked up a kernel to not do this.)  I have an incorrect
patch [1] to fix it, but I haven't gotten around to a real fix.  (I also
have stable pages reverted in my kernel.  Some day I'll submit a patch
to make it a filesystem option.  Or maybe it should even be a block
device / queue property like the alignment offset and optimal io size --
there are plenty of block device and file combinations which don't
benefit at all from stable pages.)

I'd prefer if file_update_time in page_mkwrite didn't proliferate.  A
better fix is probably to introduce a new inode flag, update it when a
page is undirtied, and then dirty and write the inode from the writeback
path.  (Kind of like my patch, but with an inode flag instead of a page
flag, and with the file_update_time done from the fs.)

[1] http://patchwork.ozlabs.org/patch/122516/

--Andy

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