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    SubjectRe: [patch] __block_write_full_page bug
    Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >
    > On Tue, 2005-04-26 at 04:50 -0700, Andrew Morton wrote:
    > > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    > > >
    > > > When running
    > > > fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
    > > > on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
    > > > page size over loopback to an image file on a tmpfs filesystem, I would
    > > > very quickly hit
    > > > BUG_ON(!buffer_async_write(bh));
    > > > in fs/buffer.c:end_buffer_async_write
    > > >
    > > > It seems that more than one request would be submitted for a given bh
    > > > at a time. __block_write_full_page looks like the culprit - with the
    > > > following patch things are very stable.
    > >
    > > What's the bug? I don't see it.
    > >
    >
    > Ah, the bug is that end_buffer_async_write first does
    > BUG_ON(!buffer_async_write(bh));
    > then a bit later does
    > clear_buffer_async_write(bh);
    >
    > That's where it was blowing up for me, because end_buffer_async_write
    > was being run twice for that buffer.
    >
    > Or did you mean *how* is it being run twice? I didn't exactly find
    > the stack traces involved, but I imagine that simply testing
    > buffer_async_write catches other requests in flight - ie. we've
    > lost track of exactly which ones we own.
    >

    How can such a thing come about? Both PageLocked() and PageWriteback() are
    supposed to stop new writeback being started against the page.

    <looks>

    Were you using nobh? I guess not. What's to stop the new
    mpage_writepage() from trying to write a page which is already under
    PageWriteback()?

    I don't think we understand this bug yet.
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