Messages in this thread | | | Date | Tue, 1 Jun 2010 17:54:17 +0200 | From | Jan Kara <> | Subject | Re: [PATCH 6/6] writeback: limit write_cache_pages integrity scanning to current EOF |
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On Thu 27-05-10 14:33:41, Andrew Morton wrote: > On Tue, 25 May 2010 20:54:12 +1000 > Dave Chinner <david@fromorbit.com> wrote: > > > From: Dave Chinner <dchinner@redhat.com> > > > > sync can currently take a really long time if a concurrent writer is > > extending a file. The problem is that the dirty pages on the address > > space grow in the same direction as write_cache_pages scans, so if > > the writer keeps ahead of writeback, the writeback will not > > terminate until the writer stops adding dirty pages. > > <looks at Jens> > > The really was a pretty basic bug. It's writeback 101 to test that case :( The code has this live-lock since Nick fixed data integrity issues in write_cache_pages which was (digging) commit 05fe478d ("mm: write_cache_pages integrity fix") in January 2009. Jens just kept the code as it was...
... > That being said, I think the patch is insufficient. If I create an > enormous (possibly sparse) file with a 16TB hole (or a run of clean > pages) in the middle and then start busily writing into that hole (run > of clean pages), the problem will still occur. > > One obvious fix for that (a) would be to add another radix-tree tag and > do two passes across the radix-tree. > > Another fix (b) would be to track the number of dirty pages per > adddress_space, and only write that number of pages. > > Another fix would be to work out how the code handled this situation > before we broke it, and restore that in some fashion. I guess fix (b) > above kinda does that. (b) does not work for data integrity sync (see changelog of the above mentioned commit). I was sending a patch doing (a) in February but in particular you raised concerns whether it's not too expensive... Since it indeed has some cost (although I was not able to measure any performance impact) and I didn't know a better solution, I just postponed the patches. But I guess it's time to revive the series and maybe we'll get further with it.
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR
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