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Date: Wed, 25 Mar 2009 16:57:21 -0700 (PDT) From: Linus Torvalds <torvalds@...ux-foundation.org> To: Theodore Tso <tytso@....edu> cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...e.hu>, Alan Cox <alan@...rguk.ukuu.org.uk>, Arjan van de Ven <arjan@...radead.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Nick Piggin <npiggin@...e.de>, Jens Axboe <jens.axboe@...cle.com>, David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Linux 2.6.29 On Wed, 25 Mar 2009, Linus Torvalds wrote: > > Yes, yes, it may need to allocate backing store (a page that was dirtied > by mmap), and I'm sure that's the reason for it all, Hmm. Thinking about that, I'm not so sure. Shouldn't that backing store allocation happen when the page is actually dirtied on ext3? I _suspect_ that goes back to the fact that ext3 is older than the "aops->set_page_dirty()" callback, and nobody taught ext3 to do the bmap's at dirty time, so now it does it at writeout time. Anyway, there we are. Old filesystems do the wrong thing (block allocation while doing writeout because they don't do it when dirtying), and newer filesystems do the wrong thing (block allocations during writeout, because they want to do delayed allocation to do the inode dirtying after doing writeback). And in either case, the VM is screwed, and can't ask for writeout, because it will be randomly throttled by the filesystem. So we do lots of async bdflush threads, which then causes IO ordering problems because now the writeout is all in random order. 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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