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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Wed, 12 Apr 2023 16:00:52 -0400
Message-ID: <CA+CK2bBVAUibUvajJP6Xvo-cYuBe57GBqZsjsc4fiKUoYmnxXQ@mail.gmail.com>
Subject: Re: [PATCH] mm: hugetlb_vmemmap: provide stronger vmemmap allocaction gurantees
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org,
        muchun.song@linux.dev, souravpanda@google.com,
        Michal Hocko <mhocko@suse.com>
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On Wed, Apr 12, 2023 at 3:57=E2=80=AFPM Mike Kravetz <mike.kravetz@oracle.c=
om> wrote:
>
> On 04/12/23 10:54, David Rientjes wrote:
> > On Wed, 12 Apr 2023, Pasha Tatashin wrote:
> >
> > > HugeTLB pages have a struct page optimizations where struct pages for=
 tail
> > > pages are freed. However, when HugeTLB pages are destroyed, the memor=
y for
> > > struct pages (vmemmap) need to be allocated again.
> > >
> > > Currently, __GFP_NORETRY flag is used to allocate the memory for vmem=
map,
> > > but given that this flag makes very little effort to actually reclaim
> > > memory the returning of huge pages back to the system can be problem.=
 Lets
> > > use __GFP_RETRY_MAYFAIL instead. This flag is also performs graceful
> > > reclaim without causing ooms, but at least it may perform a few retri=
es,
> > > and will fail only when there is genuinely little amount of unused me=
mory
> > > in the system.
> > >
> >
> > Thanks Pasha, this definitely makes sense.  We want to free the hugetlb
> > page back to the system so it would be a shame to have to strand it in =
the
> > hugetlb pool because we can't allocate the tail pages (we want to free
> > more memory than we're allocating).
>
> Agree.
>
> The hugetlb vmemmmap freeing series went through more than 20 revisions
> before being merged.  One issue with much discussion was the need to
> allocate vmemmap pages when hugetlb pages were returned to buddy.
>
> It looks like the current set of GFP flags was suggested here:
> https://lore.kernel.org/linux-mm/YC4ji+pMhtOs+KVM@dhcp22.suse.cz/
>
> Although, it was also mentioned that __GFP_RETRY_MAYFAIL could be used
> instead of __GFP_NORETRY here:
> https://lore.kernel.org/linux-mm/YCafit5ruRJ+SL8I@dhcp22.suse.cz/
>
> Adding Michal on Cc: since these were his suggestions.

Thank you for the background Mike. I have sent the 2nd version, and
added Michal into that patch.

Pasha