linux-kernel - Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE

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Date:   Tue, 17 Apr 2018 15:09:09 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Christopher Lameter <cl@...ux.com>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        Mike Snitzer <snitzer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org,
        dm-devel@...hat.com, David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE



On Tue, 17 Apr 2018, Christopher Lameter wrote:

> On Tue, 17 Apr 2018, Vlastimil Babka wrote:
> 
> > On 04/17/2018 04:45 PM, Christopher Lameter wrote:
> 
> > > But then higher order allocs are generally seen as problematic.
> >
> > I think in this case they are better than wasting/fragmenting 384kB for
> > 640kB object.
> 
> Well typically we have suggested that people use vmalloc in the past.

vmalloc is slow - it is unuseable for a buffer cache.

> > > That
> > > means that callers need to be able to tolerate failures.
> >
> > Is it any different from now? I suppose there would still be
> > smallest-order fallback involved in sl*b itself? And if your allocation
> > is so large it can fail even with the fallback (i.e. >= costly order),
> > you need to tolerate failures anyway?
> 
> Failures can occur even with < costly order as far as I can telkl. Order 0
> is the only safe one.

The alloc_pages functions seems to retry indefinitely for order <= 
PAGE_ALLOC_COSTLY_ORDER. Do you have some explanation why it should fail?

> > One corner case I see is if there is anyone who would rather use their
> > own fallback instead of the space-wasting smallest-order fallback.
> > Maybe we could map some GFP flag to indicate that.
> 
> Well if you have a fallback then maybe the slab allocator should not fall
> back on its own but let the caller deal with it.

Mikulas