Message-ID: <433fb64d-80e1-4d96-904e-10b51e40898d@redhat.com>
Date: Sat, 21 Dec 2024 15:32:11 +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, Oscar Salvador <osalvador@...e.de>,
 Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] replace free hugepage folios after migration

On 21.12.24 13:04, Ge Yang wrote:
> 
> 
> 在 2024/12/21 0:30, David Hildenbrand 写道:
>> On 20.12.24 09:56, Ge Yang wrote:
>>>
>>>
>>> 在 2024/12/20 0:40, David Hildenbrand 写道:
>>>> On 18.12.24 07:33, yangge1116@....com wrote:
>>>>> From: yangge <yangge1116@....com>
>>>>
>>>> CCing Oscar, who worked on migrating these pages during memory offlining
>>>> and alloc_contig_range().
>>>>
>>>>>
>>>>> My machine has 4 NUMA nodes, each equipped with 32GB of memory. I
>>>>> have configured each NUMA node with 16GB of CMA and 16GB of in-use
>>>>> hugetlb pages. The allocation of contiguous memory via the
>>>>> cma_alloc() function can fail probabilistically.
>>>>>
>>>>> The cma_alloc() function may fail if it sees an in-use hugetlb page
>>>>> within the allocation range, even if that page has already been
>>>>> migrated. When in-use hugetlb pages are migrated, they may simply
>>>>> be released back into the free hugepage pool instead of being
>>>>> returned to the buddy system. This can cause the
>>>>> test_pages_isolated() function check to fail, ultimately leading
>>>>> to the failure of the cma_alloc() function:
>>>>> cma_alloc()
>>>>>        __alloc_contig_migrate_range() // migrate in-use hugepage
>>>>>        test_pages_isolated()
>>>>>            __test_page_isolated_in_pageblock()
>>>>>                 PageBuddy(page) // check if the page is in buddy
>>>>
>>>> I thought this would be working as expected, at least we tested it with
>>>> alloc_contig_range / virtio-mem a while ago.
>>>>
>>>> On the memory_offlining path, we migrate hugetlb folios, but also
>>>> dissolve any remaining free folios even if it means that we will going
>>>> below the requested number of hugetlb pages in our pool.
>>>>
>>>> During alloc_contig_range(), we only migrate them, to then free them up
>>>> after migration.
>>>>
>>>> Under which circumstances doe sit apply that "they may simply be
>>>> released back into the free hugepage pool instead of being returned to
>>>> the buddy system"?
>>>>
>>>
>>> After migration, in-use hugetlb pages are only released back to the
>>> hugetlb pool and are not returned to the buddy system.
>>
>> We had
>>
>> commit ae37c7ff79f1f030e28ec76c46ee032f8fd07607
>> Author: Oscar Salvador <osalvador@...e.de>
>> Date:   Tue May 4 18:35:29 2021 -0700
>>
>>       mm: make alloc_contig_range handle in-use hugetlb pages
>>       alloc_contig_range() will fail if it finds a HugeTLB page within the
>>       range, without a chance to handle them.  Since HugeTLB pages can be
>>       migrated as any LRU or Movable page, it does not make sense to bail
>> out
>>       without trying.  Enable the interface to recognize in-use HugeTLB
>> pages so
>>       we can migrate them, and have much better chances to succeed the call.
>>
>>
>> And I am trying to figure out if it never worked correctly, or if
>> something changed that broke it.
>>
>>
>> In start_isolate_page_range()->isolate_migratepages_block(), we do the
>>
>>       ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);
>>
>> to add these folios to the cc->migratepages list.
>>
>> In __alloc_contig_migrate_range(), we migrate the pages using
>> migrate_pages().
>>
>>
>> After that, the src hugetlb folios should still be isolated?
> Yes.
> 
> But I'm
>> getting
>> confused when these pages get un-silated and putback to hugetlb/freed.
>>
> If the migration is successful, call folio_putback_active_hugetlb to
> release the src hugetlb folios back to the free hugetlb pool.
> 
> trace:
> unmap_and_move_huge_page
>       folio_putback_active_hugetlb
>           folio_put
>               free_huge_folio
> 
> alloc_contig_range_noprof
>       __alloc_contig_migrate_range
>       if (test_pages_isolated())  //to determine if hugetlb pages in buddy
>           isolate_freepages_range //grab isolated pages from freelists.
>       else
>           undo_isolate_page_range //undo isolate

Ah, now I remember, thanks.

So when we free an ordinary page, we put it onto the buddy isolate list, 
from where we can grab it later and nobody can allocate it in the meantime.

In case of hugetlb, we simply free it back to hugetlb, from where it can 
likely even get allocated immediately again.

I think that can actually happen in your proposal: the now-free page 
will get reallocated, for example for migrating the next folio. Or some 
concurrent system activity can simply allocate the now-free folio. Or am 
I missing something that prevents these now-free hugetlb folios from 
getting re-allocated after migration succeeded?


Conceptually, I think we would want migration code in the case of 
alloc_contig_range() to allocate a new folio from the buddy, and to free 
the old one back to the buddy immediately, without ever allowing 
re-allocation of it.

What needs to be handled is detecting that

(a) we want to allocate a fresh hugetlb folio as migration target
(b) if migration succeeds, we have to free the hugetlb folio back to the 
buddy
(c) if migation fails, we have to free the allocated hugetlb foliio back 
to the buddy


We could provide a custom alloc_migration_target that we pass to 
migrate_page to allocate a fresh hugetlb folio to handle (a). Using the 
put_new_folio callback we could handle (c). (b) would need some thought.

Maybe we can also just mark the source folio as we isolate it, and 
enlighten migration+freeing code to handle it automatically?

Hoping to get some feedback from hugetlb maintainers.

-- 
Cheers,

David / dhildenb

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