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    SubjectRe: MM deadlock [was: Re: arca-vm-8...]
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    >>>>> "ST" == Stephen C Tweedie <sct@redhat.com> writes:

    ST> Hi,
    ST> On Sun, 10 Jan 1999 10:35:10 -0800 (PST), Linus Torvalds
    ST> <torvalds@transmeta.com> said:

    >> On Sun, 10 Jan 1999, Stephen C. Tweedie wrote:
    >>>
    >>> Ack. I've been having a closer look, and making the superblock lock
    >>> recursive doesn't work

    >> That's fine - the superblock lock doesn't need to be re-entrant, because
    >> __GFP_IO is quite sufficient for that one.

    ST> I'm no longer convinced about that. I think it's much much worse. A
    ST> bread() on an ext2 bitmap buffer with the superblock held is only safe
    ST> if the IO can complete without _ever_ relying on a GFP_IO allocation.
    ST> That means that any interrupt allocations required in that space have to
    ST> be satisfiable by kswapd without GFP_IO, or kswapd could deadlock
    ST> on us.

    Well interrupts use GFP_ATOMIC . . .

    ST> It means that if our superblock-locked IO has to stall waiting for an
    ST> nbd server process or a raid daemon, then those daemons cannot safely do
    ST> GFP_IO. It's really gross.

    Right. And the flag not to do I/O doesn't propogate across processes.
    This sounds like a variation of the priority inheritance problem.

    I wonder if this is why there are some known deadlocks with raid?

    ST> I think it's actually ugly enough that we cannot make it safe: we can
    ST> really only be sure if we prevent all GFP_IO from any process which
    ST> might be involved in our deadlock loop, or if we avoid doing any IO with
    ST> the superblock lock held.

    ST> In fact, to make it really safe we'd need to avoid synchronous swapout
    ST> altogether: otherwise we can have

    ST> Can we get away without synchronous swapout? Notice that in this case,
    ST> kswiod may be blocked but kswapd itself will not be. As long as the nbd
    ST> server does not try to do a synchronous swap, it won't deadlock on
    ST> kswiod. In other words, it is safe to wait for avaibility of another
    ST> free page, but it is not safe to wait for completion of any single,
    ST> specific swap IO. If kswapd itself no longer performs the IO, then we
    ST> can always free more memory, until we get to the complete death stage
    ST> where there are absolutely no clean pages left in the system.

    ST> If we do this, then both the inode and the superblock deadlocks
    ST> disappear.

    Sounds good.

    I have a daemon just about ready to go, hopefully I can post it
    tommorrow for preliminary testing. It looks like my work for 2.3
    in a small part can help deadlocks after all.

    It walks the page tables and just writes out dirty pages, and marks
    them clean but it doesn't remove them from processes. So it can get
    an early jump on writing things out.

    Then if we are hitting a low memory situation (because pages become
    dirty quickly), we can just wake it up, more often.

    Currently we are doing totally asynchonous swapping but from the
    context of the process that needs memory, (so the locks are in
    different processes). Adding a second daemon will play havoc on our
    balancing but it shouldn't affect anything else.

    Grr. I forgot about sysv shm. It is the only thing doing synchronous
    swapping right now.

    Oh, and just as a side note we are currently unfairly penalizing
    threaded programs by doing for_each_task instead of for_each_mm in the
    swapout code...

    Eric

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