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Date:   Fri, 7 Jul 2023 14:12:01 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Yang Shi <shy828301@gmail.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v2 0/5] variable-order, large folios for anonymous memory
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On Fri, Jul 07, 2023 at 01:40:53PM +0200, David Hildenbrand wrote:
> On 06.07.23 10:02, Ryan Roberts wrote:
> But can you comment on the page migration part (IOW did you try it already)?
> 
> For example, memory hotunplug, CMA, MCE handling, compaction all rely on
> page migration of something that was allocated using GFP_MOVABLE to actually
> work.
> 
> Compaction seems to skip any higher-order folios, but the question is if the
> udnerlying migration itself works.
> 
> If it already works: great! If not, this really has to be tackled early,
> because otherwise we'll be breaking the GFP_MOVABLE semantics.

I have looked at this a bit.  _Migration_ should be fine.  _Compaction_
is not.

If you look at a function like folio_migrate_mapping(), it all seems
appropriately folio-ised.  There might be something in there that is
slightly wrong, but that would just be a bug to fix, not a huge
architectural problem.

The problem comes in the callers of migrate_pages().  They pass a
new_folio_t callback.  alloc_migration_target() is the usual one passed
and as far as I can tell is fine.  I've seen no problems reported with it.

compaction_alloc() is a disaster, and I don't know how to fix it.
The compaction code has its own allocator which is populated with order-0
folios.  How it populates that freelist is awful ... see split_map_pages()

> Is swapping working as expected? zswap?

Suboptimally.  Swap will split folios in order to swap them.  Somebody
needs to fix that, but it should work.