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    SubjectRe: [PATCH] scsi: fix sense_slab/bio swapping livelock
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    On Mon, 2008-04-07 at 21:31 +0100, Hugh Dickins wrote:
    > 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.

    Right, but I doubt we'd ever get something like that merged though -
    esp. as it will basically destroy the SLUB fast-path.

    SLAB allocation fairness:
    http://lkml.org/lkml/2007/1/16/61

    I abandoned this approach because it was too expensive; it was reduced
    to the ALLOC_NO_WATERMARKS state transition. Which is much more unlikely
    to happen and it's generally accepted we're in a slow path once we
    really dive so low into the reserves.

    The latest posting:
    http://lkml.org/lkml/2008/3/20/214




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