linux-kernel - Re: [PATCH] mm: hugetlb_vmemmap: provide stronger vmemmap allocaction gurantees

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Date:   Wed, 12 Apr 2023 16:00:52 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     David Rientjes <rientjes@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        muchun.song@...ux.dev, souravpanda@...gle.com,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: hugetlb_vmemmap: provide stronger vmemmap allocaction gurantees

On Wed, Apr 12, 2023 at 3:57 PM Mike Kravetz <mike.kravetz@...cle.com> 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 memory for
> > > struct pages (vmemmap) need to be allocated again.
> > >
> > > Currently, __GFP_NORETRY flag is used to allocate the memory for vmemmap,
> > > 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 retries,
> > > and will fail only when there is genuinely little amount of unused memory
> > > 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