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Date: Tue, 17 Apr 2018 14:53:45 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
Subject: Re: slab: introduce the flag SLAB_MINIMIZE_WASTE
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To: Christopher Lameter <cl@linux.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Mike Snitzer <snitzer@redhat.com>, Matthew Wilcox <willy@infradead.org>, Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org, dm-devel@redhat.com, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org



On Tue, 17 Apr 2018, Christopher Lameter wrote:

> On Mon, 16 Apr 2018, Mikulas Patocka wrote:
> 
> > dm-bufio deals gracefully with allocation failure, because it preallocates
> > some buffers with vmalloc, but other subsystems may not deal with it and
> > they cound return ENOMEM randomly or misbehave in other ways. So, the
> > "SLAB_MINIMIZE_WASTE" flag is also saying that the allocation may fail and
> > the caller is prepared to deal with it.
> >
> > The slub subsystem does actual fallback to low-order when the allocation
> > fails (it allows different order for each slab in the same cache), but
> > slab doesn't fallback and you get NULL if higher-order allocation fails.
> > So, SLAB_MINIMIZE_WASTE is needed for slab because it will just randomly
> > fail with higher order.
> 
> Fix Slab instead of adding a flag that is only useful for one allocator?

Slab assumes that all slabs have the same order, so it's not so easy to 
fix it.

Mikulas