Re: [PATCH] scsi: fix sense_slab/bio swapping livelock

[Peter Zijlstra]

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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