linux-kernel - Re: [PATCH] replace free hugepage folios after migration

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Message-ID: <fc1241a6-6760-4f73-840d-4f3a538644aa@redhat.com>
Date: Wed, 8 Jan 2025 22:05:18 +0100
From: David Hildenbrand <david@...hat.com>
To: Ge Yang <yangge1116@....com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 21cnbao@...il.com, baolin.wang@...ux.alibaba.com, muchun.song@...ux.dev,
 liuzixing@...on.cn
Subject: Re: [PATCH] replace free hugepage folios after migration

Sorry for the late reply, holidays ...

>> Did you ever try allocating a larger range with a single
>> alloc_contig_range() call, that possibly has to migrate multiple hugetlb
>> folios in one go (and maybe just allocates one of the just-freed hugetlb
>> folios as migration target)?
>>
> 
> I have tried using a single alloc_contig_range() call to allocate a
> larger contiguous range, and it works properly. This is because during
> the period between __alloc_contig_migrate_range() and
> isolate_freepages_range(), no one allocates a hugetlb folio from the
> free hugetlb pool.

Did you trigger the following as well?

alloc_contig_range() that covers multiple in-use hugetlb pages, like

[ huge 0 ] [ huge 1 ] [ huge 2 ] [ huge 3 ]

I assume the following happens:

To migrate huge 0, we have to allocate a fresh page from the buddy. 
After migration, we return now-free huge 0 to the pool.

To migrate huge 1, we can just grab now-free huge 0 from the pool, and 
not allocate a fresh one from the buddy.

At least that's my impression when reading 
alloc_migration_target()->alloc_hugetlb_folio_nodemask().

Or is for some reason available_huge_pages()==false and we always end up 
in alloc_migrate_hugetlb_folio()->alloc_fresh_hugetlb_folio()?

Sorry for the stupid questions, the code is complicated, and I cannot 
see how this would work.

-- 
Cheers,

David / dhildenb