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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 29 Jan 2021 14:16:19 +0800
Message-ID: <CAMZfGtW7Bc-QhjW7MJyVwYXPEihbKyAE20NLgs3mVmgALh_X2A@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v13 05/12] mm: hugetlb: allocate the
 vmemmap pages associated with each HugeTLB page
To:     Oscar Salvador <osalvador@suse.de>
Cc:     David Hildenbrand <david@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com,
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        Peter Zijlstra <peterz@infradead.org>, viro@zeniv.linux.org.uk,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
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On Fri, Jan 29, 2021 at 6:29 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Wed, Jan 27, 2021 at 11:36:15AM +0100, David Hildenbrand wrote:
> > Extending on that, I just discovered that only x86-64, ppc64, and arm64
> > really support hugepage migration.
> >
> > Maybe one approach with the "magic switch" really would be to disable
> > hugepage migration completely in hugepage_migration_supported(), and
> > consequently making hugepage_movable_supported() always return false.
>
> Ok, so migration would not fork for these pages, and since them would
> lay in !ZONE_MOVABLE there is no guarantee we can unplug the memory.
> Well, we really cannot unplug it unless the hugepage is not used
> (it can be dissolved at least).
>
> Now to the allocation-when-freeing.
> Current implementation uses GFP_ATOMIC(or wants to use) + forever loop.
> One of the problems I see with GFP_ATOMIC is that gives you access
> to memory reserves, but there are more users using those reserves.
> Then, worst-scenario case we need to allocate 16MB order-0 pages
> to free up 1GB hugepage, so the question would be whether reserves
> really scale to 16MB + more users accessing reserves.
>
> As I said, if anything I would go for an optimistic allocation-try
> , if we fail just refuse to shrink the pool.
> User can always try to shrink it later again via /sys interface.

Yeah. It seems that this is the easy way to move on.

Thanks.

>
> Since hugepages would not be longer in ZONE_MOVABLE/CMA and are not
> expected to be migratable, is that ok?
>
> Using the hugepage for the vmemmap array was brought up several times,
> but that would imply fragmenting memory over time.
>
> All in all seems to be overly complicated (I might be wrong).
>
>
> > Huge pages would never get placed onto ZONE_MOVABLE/CMA and cannot be
> > migrated. The problem I describe would apply (careful with using
> > ZONE_MOVABLE), but well, it can at least be documented.
>
> I am not a page allocator expert but cannot the allocation fallback
> to ZONE_MOVABLE under memory shortage on other zones?
>
>
> --
> Oscar Salvador
> SUSE L3