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SubjectRe: [PATCH] scsi: fix sense_slab/bio swapping livelock
On Sun, 2008-04-06 at 23:56 +0100, Hugh Dickins wrote:
> Since 2.6.25-rc7, I've been seeing an occasional livelock on one
> x86_64 machine, copying kernel trees to tmpfs, paging out to swap.
> Signature: 6000 pages under writeback but never getting written;
> most tasks of interest trying to reclaim, but each get_swap_bio
> waiting for a bio in mempool_alloc's io_schedule_timeout(5*HZ);
> every five seconds an atomic page allocation failure report from
> kblockd failing to allocate a sense_buffer in __scsi_get_command.
> __scsi_get_command has a (one item) free_list to protect against
> this, but rc1's [SCSI] use dynamically allocated sense buffer
> de25deb18016f66dcdede165d07654559bb332bc upset that slightly.
> When it fails to allocate from the separate sense_slab, instead
> of giving up, it must fall back to the command free_list, which
> is sure to have a sense_buffer attached.
> Either my earlier -rc testing missed this, or there's some recent
> contributory factor. One very significant factor is SLUB, which
> merges slab caches when it can, and on 64-bit happens to merge
> both bio cache and sense_slab cache into kmalloc's 128-byte cache:
> so that under this swapping load, bios above are liable to gobble
> up all the slots needed for scsi_cmnd sense_buffers below.
> That's disturbing behaviour, and I tried a few things to fix it.
> Adding a no-op constructor to the sense_slab inhibits SLUB from
> merging it, and stops all the allocation failures I was seeing;
> but it's rather a hack, and perhaps in different configurations
> we have other caches on the swapout path which are ill-merged.
> Another alternative is to revert the separate sense_slab, using
> cache-line-aligned sense_buffer allocated beyond scsi_cmnd from
> the one kmem_cache; but that might waste more memory, and is
> only a way of diverting around the known problem.
> While I don't like seeing the allocation failures, and hate the
> idea of all those bios piled up above a scsi host working one by
> one, it does seem to emerge fairly soon with the livelock fix.
> So lacking better ideas, stick with that one clear fix for now.
> Signed-off-by: Hugh Dickins <>

This was sort of accidentally fixed in scsi-misc by commit

commit c5f73260b289cb974928eac05f2d84e58ddfc020
Author: James Bottomley <>
Date: Thu Mar 13 11:16:33 2008 -0500

[SCSI] consolidate command allocation in a single place

Could you check that:

and see if it alleviates the problem? ... if so, we can work out which
pieces to backport.



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