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Subject[PATCH] scsi: fix sense_slab/bio swapping livelock
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 <hugh@veritas.com>
---
drivers/scsi/scsi.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)
--- 2.6.25-rc8/drivers/scsi/scsi.c 2008-03-05 10:47:40.000000000 +0000
+++ linux/drivers/scsi/scsi.c 2008-04-05 22:23:40.000000000 +0100
@@ -181,6 +181,18 @@ struct scsi_cmnd *__scsi_get_command(str
cmd = kmem_cache_alloc(shost->cmd_pool->cmd_slab,
gfp_mask | shost->cmd_pool->gfp_mask);

+ if (likely(cmd)) {
+ buf = kmem_cache_alloc(shost->cmd_pool->sense_slab,
+ gfp_mask | shost->cmd_pool->gfp_mask);
+ if (likely(buf)) {
+ memset(cmd, 0, sizeof(*cmd));
+ cmd->sense_buffer = buf;
+ } else {
+ kmem_cache_free(shost->cmd_pool->cmd_slab, cmd);
+ cmd = NULL;
+ }
+ }
+
if (unlikely(!cmd)) {
unsigned long flags;

@@ -197,16 +209,6 @@ struct scsi_cmnd *__scsi_get_command(str
memset(cmd, 0, sizeof(*cmd));
cmd->sense_buffer = buf;
}
- } else {
- buf = kmem_cache_alloc(shost->cmd_pool->sense_slab,
- gfp_mask | shost->cmd_pool->gfp_mask);
- if (likely(buf)) {
- memset(cmd, 0, sizeof(*cmd));
- cmd->sense_buffer = buf;
- } else {
- kmem_cache_free(shost->cmd_pool->cmd_slab, cmd);
- cmd = NULL;
- }
}

return cmd;

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