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Date: Thu, 19 Mar 2009 17:34:06 -0700 From: Ying Han <yinghan@...gle.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, guichaz@...il.com, Alex Khesin <alexk@...gle.com>, Mike Waychison <mikew@...gle.com>, Rohit Seth <rohitseth@...gle.com>, Nick Piggin <nickpiggin@...oo.com.au>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: ftruncate-mmap: pages are lost after writing to mmaped file. On Wed, Mar 18, 2009 at 3:40 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > > > On Wed, 18 Mar 2009, Andrew Morton wrote: > >> On Wed, 18 Mar 2009 12:44:08 -0700 Ying Han <yinghan@...gle.com> wrote: >> > >> > The "bad pages" count differs each time from one digit to 4,5 digit >> > for 128M ftruncated file. and what i also found that the bad page >> > number are contiguous for each segment which total bad pages container >> > several segments. ext "1-4, 9-20, 48-50" ( batch flushing ? ) > > Yeah, probably the batched write-out. > > Can you say what filesystem, and what mount-flags you use? Iirc, last time > we had MAP_SHARED lost writes it was at least partly triggered by the > filesystem doing its own flushing independently of the VM (ie ext3 with > "data=journal", I think), so that kind of thing does tend to matter. > > See for example commit ecdfc9787fe527491baefc22dce8b2dbd5b2908d. > >> > (The failure is reproduced based on 2.6.29-rc8, also happened on >> > 2.6.18 kernel. . Here is the simple test case to reproduce it with >> > memory pressure. ) >> >> Thanks. This will be a regression - the testing I did back in the days >> when I actually wrote stuff would have picked this up. >> >> Perhaps it is a 2.6.17 thing. Which, IIRC, is when we made the changes to >> redirty pages on each write fault. Or maybe it was something else. > > Hmm. I _think_ that changes went in _after_ 2.6.18, if you're talking > about Peter's exact dirty page tracking. If I recall correctly, that > became then 2.6.19, and then had the horrible mm dirty bit loss that > triggered in librtorrent downloads, which got fixed sometime after 2.6.20 > (and back-ported). > > So if 2.6.18 shows the same problem, then it's a _really_ old bug, and not > related to the exact dirty tracking. > > The exact dirty accounting patch I'm talking about is d08b3851da41 ("mm: > tracking shared dirty pages"), but maybe you had something else in mind? > >> Given the amount of time for which this bug has existed, I guess it isn't a >> 2.6.29 blocker, but once we've found out the cause we should have a little >> post-mortem to work out how a bug of this nature has gone undetected for so >> long. > > I'm somewhat surprised, because this test-program looks like a very simple > version of the exact one that I used to track down the 2.6.20 mmap > corruption problems. And that one got pretty heavily tested back then, > when people were looking at it (December 2006) and then when trying out my > fix for it. > > Ying Han - since you're all set up for testing this and have reproduced it > on multiple kernels, can you try it on a few more kernel versions? It > would be interesting to both go further back in time (say 2.6.15-ish), > _and_ check something like 2.6.21 which had the exact dirty accounting > fix. Maybe it's not really an old bug - maybe we re-introduced a bug that > was fixed for a while. I tried 2.6.24 for couple of hours and the problem not happening yet. While the same test on 2.6.25, the problem happen right away. > > Linus > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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