Re: [patch 9/9] mm: fix pagecache write deadlocks

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Tue, 2007-02-06 at 03:09 +0100, Nick Piggin wrote:
> On Sun, Feb 04, 2007 at 05:40:35PM +0000, Anton Altaparmakov wrote:
> > On Sun, 4 Feb 2007, Andrew Morton wrote:
> > > truncate's OK: we're holding i_mutex.
> > 
> > How about excluding readpage() (in addition to truncate if Nick is right  
> > and some cases of truncate do not hold i_mutex) with an extra page flag as
> > I proposed for truncate exclusion?  Then it would not matter that
> > prepare_write might have allocated blocks and might expose stale data.    
> > It would go to sleep and wait on the bit to be cleared instead of trying  
> > to bring the page uptodate.  It can then lock the page and either find it 
> > uptodate (because commit_write did it) or not and then bring it uptodate.
> > 
> > Then we could safely fault in the page, copy from it into a temporary 
> > page, then lock the destination page again and copy into it.
> > 
> > This is getting more involved as a patch again...  )-:  But at least it   
> > does not affect the common case except for having to check the new page 
> > flag in every readpage() and truncate() call.  But at least the checks 
> > could be with an "if (unlikely(newpageflag()))" so should not be too bad.
> > 
> > Have I missed anything this time?
> 
> Yes. If you have a flag to exclude readpage(), then you must also
> exclude filemap_nopage, in which case it is still deadlocky.

Ouch, you are of course right.  )-:

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/