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    SubjectRe: [PATCH] scsi: fix sense_slab/bio swapping livelock
    On Mon, 7 Apr 2008, Peter Zijlstra wrote:
    > On Mon, 2008-04-07 at 20:40 +0100, Hugh Dickins wrote:
    > >
    > > My supposition is that once a page has been allocated from __GFP_HIGH
    > > reserves to a scsi sense_slab, swap_writepages are liable to gobble up
    > > the rest of the page with bio allocations which they wouldn't have had
    > > access to traditionally (i.e. under SLAB).
    > >
    > > So an unexpected behaviour emerges from SLUB's slab merging.
    > Somewhere along the line of my swap over network patches I made
    > 'robustified' SLAB to ensure these sorts of things could not happen - it
    > came at a cost though.
    > It would basically fail[*] allocations that had a higher low watermark
    > than what was used to allocate the current slab.
    > [*] - well, it would attempt to allocate a new slab to raise the current
    > watermark, but failing that it would fail the allocation.

    Thanks, Peter: that sounds just right to me; but a larger change than
    we'd want to jump into for this one particular issue - it might have
    its own unexpected consequences.

    > > If we had a SLAB_NOMERGE flag, would we want to apply it to the
    > > bio cache or to the scsi_sense_cache or to both? My difficulty
    > > in answering that makes me wonder whether such a flag is right.
    > If this is critical to avoid memory deadlocks, I would suggest using
    > mempools (or my reserve framework).

    No, the critical part of it has been dealt with (small fix to scsi
    free_list handling: which resembles a mempool, but done its own way).

    What remains is about "unsightly" behaviour, the system having a
    tendency to collapse briefly into far-from-efficient operation
    when out of memory.


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