Re: [PATCH] SLUB: Don't drop __GFP_NOFAIL completely fromallocate_slab() (was: Re: [Bug #14265] ifconfig: page allocationfailure. order:5,ode:0x8020 w/ e100)

[Karol Lewandowski]

On Wed, Oct 21, 2009 at 02:06:41PM -0700, David Rientjes wrote:
> On Wed, 21 Oct 2009, Karol Lewandowski wrote:
> 
> > commit d6849591e042bceb66f1b4513a1df6740d2ad762
> > Author: Karol Lewandowski <karol.k.lewandowski@xxxxxxxxx>
> > Date:   Wed Oct 21 21:01:20 2009 +0200
> > 
> >     SLUB: Don't drop __GFP_NOFAIL completely from allocate_slab()
> >     
> >     Commit ba52270d18fb17ce2cf176b35419dab1e43fe4a3 unconditionally
> >     cleared __GFP_NOFAIL flag on all allocations.
> >     
> 
> No, it clears __GFP_NOFAIL from the first allocation of oo_order(s->oo).  
> If that fails (and it's easy to fail, it has __GFP_NORETRY), another 
> allocation is attempted with oo_order(s->min), for which __GFP_NOFAIL 
> would be preserved if that's the slab cache's allocflags.

Right, patch is junk.

However, I haven't been able to trigger failures since I've switched
to SLAB allocator.  That patch seemed related (and wrong), but it
wasn't.

> >  		 */
> > -		page = alloc_slab_page(flags, node, oo);
> > +		page = alloc_slab_page(flags | nofail, node, oo);
> >  		if (!page)
> >  			return NULL;
> >  
> > 
> 
> This does nothing.  You may have missed that the lower order allocation is 
> passing 'flags' (which is a union of the gfp flags passed to 
> allocate_slab() based on the allocation context and the cache's 
> allocflags), and not alloc_gfp where __GFP_NOFAIL is masked.

Right, I missed that.

> Nack.
> 
> Note: slub isn't going to be a culprit in order 5 allocation failures 
> since they have kmalloc passthrough to the page allocator.

However, it might change fragmentation somewhat I guess.  This might
make problem more/less visible.
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