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Date:   Thu, 13 Apr 2023 20:12:16 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mike.kravetz@oracle.com, muchun.song@linux.dev,
        rientjes@google.com, souravpanda@google.com
Subject: Re: [PATCH v2] mm: hugetlb_vmemmap: provide stronger vmemmap
 allocation guarantees
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On Thu 13-04-23 13:11:39, Pavel Tatashin wrote:
> On Thu, Apr 13, 2023 at 11:25 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 13-04-23 11:05:20, Pavel Tatashin wrote:
[...]
> > > This is a theoretical concern. Freeing a 1G page requires 16M of free
> > > memory. A machine might need to be reconfigured from one task to
> > > another, and release a large number of 1G pages back to the system if
> > > allocating 16M fails, the release won't work.
> >
> > This is really an important "detail" changelog should mention. While I
> > am not really against that change I would much rather see that as a
> > result of a real world fix rather than a theoretical concern. Mostly
> > because a real life scenario would allow us to test the
> > __GFP_RETRY_MAYFAIL effectivness. As that request might fail as well we
> > just end up with a theoretical fix for a theoretical problem. Something
> > that is easy to introduce but much harder to get rid of should we ever
> > need to change __GFP_RETRY_MAYFAIL implementation for example.
> 
> I will add this to changelog in v3. If  __GFP_RETRY_MAYFAIL is
> ineffective we will receive feedback once someone hits this problem.

I do not remember anybody hitting this with the current __GFP_NORETRY.
So arguably there is nothing to be fixed ATM.

> Otherwise, we will never hear about it. I think overall it is safer to
> keep this code with __GFP_RETRY_MAYFAIL flag.
> 
> >
> > > In an ideal scenario we should guarantee that this never fails: that
> > > we always can free HugeTLB pages back to the system. At the very least
> > > we could steal the memory for vmemmap from the page that is being
> > > released.
> >
> > Yes, this really bothered me when the concept was introduced initially.
> > I am always concerned when you need to allocate in order to free memory.
> > Practically speaking we haven't heard about bug reports so maybe this is
> > not such a big deal as I thought.
> 
> I suspect this is because at the moment it is not that frequent when a
> machine is reconfigured from having a lot of HugeTLB based workload to
> non-HugeTLB workload.

Yes, hugetlb workloads tend to be pretty static from my experience.
-- 
Michal Hocko
SUSE Labs