Re: [PATCHv5] mm: skip CMA pages when they are not available

[David Hildenbrand]

On 12.06.23 11:35, Zhaoyang Huang wrote:
> On Mon, Jun 12, 2023 at 5:29 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 10.06.23 00:35, Andrew Morton wrote:
> > > On Wed, 31 May 2023 10:51:01 +0800 "zhaoyang.huang" <zhaoyang.huang@xxxxxxxxxx> wrote:
> > >
> > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > >
> > > > This patch fixes unproductive reclaiming of CMA pages by skipping them when they
> > > > are not available for current context. It is arise from bellowing OOM issue, which
> > > > caused by large proportion of MIGRATE_CMA pages among free pages.
> > > >
> > > > [   36.172486] [03-19 10:05:52.172] ActivityManager: page allocation failure: order:0, mode:0xc00(GFP_NOIO), nodemask=(null),cpuset=foreground,mems_allowed=0
> > > > [   36.189447] [03-19 10:05:52.189] DMA32: 0*4kB 447*8kB (C) 217*16kB (C) 124*32kB (C) 136*64kB (C) 70*128kB (C) 22*256kB (C) 3*512kB (C) 0*1024kB 0*2048kB 0*4096kB = 35848kB
> > > > [   36.193125] [03-19 10:05:52.193] Normal: 231*4kB (UMEH) 49*8kB (MEH) 14*16kB (H) 13*32kB (H) 8*64kB (H) 2*128kB (H) 0*256kB 1*512kB (H) 0*1024kB 0*2048kB 0*4096kB = 3236kB
> > > > ...
> > > > [   36.234447] [03-19 10:05:52.234] SLUB: Unable to allocate memory on node -1, gfp=0xa20(GFP_ATOMIC)
> > > > [   36.234455] [03-19 10:05:52.234] cache: ext4_io_end, object size: 64, buffer size: 64, default order: 0, min order: 0
> > > > [   36.234459] [03-19 10:05:52.234] node 0: slabs: 53,objs: 3392, free: 0
> > >
> > > We saw plenty of feedback for earlier versions, but now silence.  Does
> > > this mean we're all OK with v5?
> >
> > The logic kind-of makes sense to me (but the kswapd special-casing
> > already shows that it might be a bit fragile for future use), but I did
> > not yet figure out if this actually fixes something or is a pure
> > performance improvement.
> >
> > As we phrased it in the comment "It is waste of effort", but in the
> > patch description "This patch fixes unproductive reclaiming" + a scary
> > dmesg.
> >
> > Am I correct that this is a pure performance optimization (and the issue
> > revealed itself in that OOM report), or does this actually *fix* something?
> >
> > If it's a performance improvement, it would be good to show that it is
> > an actual improvement worth the churn ...
> Sorry for the confusion. As for the OOM issue, the previous
> commit(https://lkml.kernel.org/r/1683782550-25799-1-git-send-email-zhaoyang.huang@xxxxxxxxxx)
> helps to decrease the fail rate from 12/20 to 2/20, which it turn to
> be 0 when applying this patch.

Thanks! Can we make that clearer in the patch description? I'm 
struggling a bit my self to find the right words.

Something like

"This change further decreases the chance for wrong OOMs in the presence 
of a lot of CMA memory."

?

In any case

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb