Messages in this thread |  | | Date | Mon, 7 Apr 2008 21:31:54 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH] scsi: fix sense_slab/bio swapping livelock |
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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.
Hugh
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