linux-kernel - Re: [RFC PATCH 3/3] hugetlbfs: don't retry when pool page allocations start to fail

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Message-ID: <20190725224307.GE2708@suse.de>
Date:   Thu, 25 Jul 2019 23:43:07 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 3/3] hugetlbfs: don't retry when pool page
 allocations start to fail

On Thu, Jul 25, 2019 at 10:15:29AM -0700, Mike Kravetz wrote:
> On 7/25/19 1:13 AM, Mel Gorman wrote:
> > On Wed, Jul 24, 2019 at 10:50:14AM -0700, Mike Kravetz wrote:
> >> When allocating hugetlbfs pool pages via /proc/sys/vm/nr_hugepages,
> >> the pages will be interleaved between all nodes of the system.  If
> >> nodes are not equal, it is quite possible for one node to fill up
> >> before the others.  When this happens, the code still attempts to
> >> allocate pages from the full node.  This results in calls to direct
> >> reclaim and compaction which slow things down considerably.
> >>
> >> When allocating pool pages, note the state of the previous allocation
> >> for each node.  If previous allocation failed, do not use the
> >> aggressive retry algorithm on successive attempts.  The allocation
> >> will still succeed if there is memory available, but it will not try
> >> as hard to free up memory.
> >>
> >> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> > 
> > set_max_huge_pages can fail the NODEMASK_ALLOC() alloc which you handle
> > *but* in the event of an allocation failure this bug can silently recur.
> > An informational message might be justified in that case in case the
> > stall should recur with no hint as to why.
> 
> Right.
> Perhaps a NODEMASK_ALLOC() failure should just result in a quick exit/error.
> If we can't allocate a node mask, it is unlikely we will be able to allocate
> a/any huge pages.  And, the system must be extremely low on memory and there
> are likely other bigger issues.
> 

That might be better overall, you make a valid point that a failed
kmalloc is not a good sign for hugetlbfs allocations.

> There have been discussions elsewhere about discontinuing the use of
> NODEMASK_ALLOC() and just putting the mask on the stack.  That may be
> acceptable here as well.
> 

They can be big and while this particular path would be relatively safe,
I think the fact that there will not be much functional difference
between allocating on the stack and a failed kmalloc in terms of
hugetlbfs allocation success rates.

> >                                            Technically passing NULL into
> > NODEMASK_FREE is also safe as kfree (if used for that kernel config) can
> > handle freeing of a NULL pointer. However, that is cosmetic more than
> > anything. Whether you decide to change either or not;
> 
> Yes.
> I will clean up with an updated series after more feedback.
> 

Thanks.

-- 
Mel Gorman
SUSE Labs