Messages in this thread | | | Subject | Re: [PATCH] scsi: fix sense_slab/bio swapping livelock | From | Peter Zijlstra <> | Date | Mon, 07 Apr 2008 22:47:13 +0200 |
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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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