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Date:   Wed, 3 Aug 2022 11:52:10 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Baoquan He <bhe@redhat.com>,
        John Donnelly <john.p.donnelly@oracle.com>,
        David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] dma/pool: do not complain if DMA pool is not allocated
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 <20220325164856.GA16800@lst.de>
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On Fri 25-03-22 17:54:33, Michal Hocko wrote:
> On Fri 25-03-22 17:48:56, Christoph Hellwig wrote:
> > On Fri, Mar 25, 2022 at 01:58:42PM +0100, Michal Hocko wrote:
> > > Dang, I have just realized that I have misread the boot log and it has
> > > turned out that a674e48c5443 is covering my situation because the
> > > allocation failure message says:
> > >
> > > Node 0 DMA free:0kB boost:0kB min:0kB low:0kB high:0kB reserved_highatomic:0KB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:636kB managed:0kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
> > 
> > As in your report is from a kernel that does not have a674e48c5443
> > yet?
> 
> yes. I just mixed up the early boot messages and thought that DMA zone
> ended up with a single page. That message was saying something else
> though.

OK, so I have another machine spewing this warning. Still on an older
kernel but I do not think the current upstream would be any different in
that regards. This time the DMA zone is populated and consumed from
large part and the pool size request is just too large for it:

[   14.017417][    T1] swapper/0: page allocation failure: order:10, mode:0xcc1(GFP_KERNEL|GFP_DMA), nodemask=(null),cpuset=/,mems_allowed=0-7
[   14.017429][    T1] CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.14.21-150400.22-default #1 SLE15-SP4 0b6a6578ade2de5c4a0b916095dff44f76ef1704
[   14.017434][    T1] Hardware name: XXXX
[   14.017437][    T1] Call Trace:
[   14.017444][    T1]  <TASK>
[   14.017449][    T1]  dump_stack_lvl+0x45/0x57
[   14.017469][    T1]  warn_alloc+0xfe/0x160
[   14.017490][    T1]  __alloc_pages_slowpath.constprop.112+0xc27/0xc60
[   14.017497][    T1]  ? rdinit_setup+0x2b/0x2b
[   14.017509][    T1]  ? rdinit_setup+0x2b/0x2b
[   14.017512][    T1]  __alloc_pages+0x2d5/0x320
[   14.017517][    T1]  alloc_page_interleave+0xf/0x70
[   14.017531][    T1]  atomic_pool_expand+0x4a/0x200
[   14.017541][    T1]  ? rdinit_setup+0x2b/0x2b
[   14.017544][    T1]  __dma_atomic_pool_init+0x44/0x90
[   14.017556][    T1]  dma_atomic_pool_init+0xad/0x13f
[   14.017560][    T1]  ? __dma_atomic_pool_init+0x90/0x90
[   14.017562][    T1]  do_one_initcall+0x41/0x200
[   14.017581][    T1]  kernel_init_freeable+0x236/0x298
[   14.017589][    T1]  ? rest_init+0xd0/0xd0
[   14.017596][    T1]  kernel_init+0x16/0x120
[   14.017599][    T1]  ret_from_fork+0x22/0x30
[   14.017604][    T1]  </TASK>
[...]
[   14.018026][    T1] Node 0 DMA free:160kB boost:0kB min:0kB low:0kB high:0kB reserved_highatomic:0KB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:15996kB managed:15360kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
[   14.018035][    T1] lowmem_reserve[]: 0 0 0 0 0
[   14.018339][    T1] Node 0 DMA: 0*4kB 0*8kB 0*16kB 1*32kB (U) 0*64kB 1*128kB (U) 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 160kB

So the DMA zone has only 160kB free while the pool would like to use 4MB
of it which obviously fails. I haven't tried to check who is consuming
the DMA zone memory and why but this shouldn't be all that important
because the pool clearly cannot allocate and there is not much the
user/admin can do about that. Well, the pool could be explicitly
requested smaller but is that really what we expect them to do?
  
> > > I thought there are only few pages in the managed by the DMA zone. This
> > > is still theoretically possible so I think __GFP_NOWARN makes sense here
> > > but it would require to change the patch description.
> > > 
> > > Is this really worth it?
> > 
> > In general I think for kernels where we need the pool and can't allocate
> > it, a warning is very useful.  We just shouldn't spew it when there is
> > no need for the pool to start with.
> 
> Well, do we have any way to find that out during early boot?

Thinking about it. We should get a warning when the actual allocation
from the pool fails no? That would be more useful information than the
pre-allocation failure when it is not really clear whether anybody is
ever going to consume it.

What do you think? Should I repost my original patch with the updated
changelog?
-- 
Michal Hocko
SUSE Labs