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Date: Mon, 18 Dec 2006 20:30:12 +1100 From: Nick Piggin <nickpiggin@...oo.com.au> To: Andrew Morton <akpm@...l.org> CC: Linus Torvalds <torvalds@...l.org>, andrei.popa@...eo.ro, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Hugh Dickins <hugh@...itas.com>, Florian Weimer <fw@...eb.enyo.de>, Marc Haber <mh+linux-kernel@...schlus.de>, Martin Michlmayr <tbm@...ius.com> Subject: Re: 2.6.19 file content corruption on ext3 Andrew Morton wrote: > On Sun, 17 Dec 2006 21:50:43 -0800 (PST) > Linus Torvalds <torvalds@...l.org> wrote: > > >> >>On Mon, 18 Dec 2006, Nick Piggin wrote: >> >>>I can't see how that's exactly a problem -- so long as the page does not >>>get reclaimed (it won't, because we have a ref on it) then all that matters >>>is that the page eventually gets marked dirty. >> >>But the point being that "try_to_free_buffers()" marks it clean >>AFTERWARDS. >> >>So yes, the page gets marked dirty in the pte's - the hardware generally >>does that for us, so we don't have to worry about that part going on. >> >>But "try_to_free_buffers()" seems to clear those dirty bits without >>serializing it really any way. It just says "ok, I will now clear them". >>Without knowing whether the dirty bits got set before the IO that cleared >>the buffer head dirty bits or not. > > > Yes, I can't see anything correct about the current behaviour. > > But I'm going blue in the face here trying to feed try_to_free_buffers() a > page_mapped(page), without success. pagevec_strip() presumably isn't > triggering. I can trigger it here, with a kernel patch to call pagevec_strip unconditionally. I am seeing it clearing pte dirty bits, which is surely a dataloss bug. BUG: warning at mm/page-writeback.c:862/clear_page_dirty_warn() [<c013f65a>] clear_page_dirty_warn+0xdb/0xdd [<c0174309>] try_to_free_buffers+0x6b/0x7e [<c01937ec>] ext3_releasepage+0x0/0x74 [<c013bb48>] try_to_release_page+0x2c/0x40 [<c0140925>] pagevec_strip+0x52/0x54 [<c0141580>] shrink_active_list+0x2a0/0x3c8 [<c0142100>] shrink_zone+0xcd/0xea [<c014266d>] kswapd+0x311/0x41e [<c012c6aa>] autoremove_wake_function+0x0/0x37 [<c014235c>] kswapd+0x0/0x41e [<c012c527>] kthread+0xde/0xe2 [<c012c449>] kthread+0x0/0xe2 [<c010395b>] kernel_thread_helper+0x7/0x1c ======================= (clear_page_dirty_warn() is test_clear_page_dirty which WARN_ON()s the result of page_mkclean) > This will (at least) cause truncate to do peculiar things. > do_invalidatepage() runs discard_buffer() against the dirty page and will > then expect try_to_free_buffers() to remove those buffers and then clean > the page. truncate_complete_page() will clean the page, but it still has > those invalidated buffers. We'll end up with a large number of clean, > unused pages on the LRU, with attached buffers. These should eventually > get reaped, but it'll change the page aging dynamics. This isn't so nice. I wonder if you could just ClearPageDirty before calling try_to_free_buffers in this case, or is that too much of a hack? Ideally I guess you want a variant that is happy to discard dirtiness (alternatively, my proposal to redirty the page if we find a dirty pte should also handle this). -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com - 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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