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Date: Tue, 27 Feb 2024 16:01:01 +0100
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: Matthew Wilcox <willy@...radead.org>,
Charan Teja Kalla <quic_charante@...cinc.com>, gregkh@...uxfoundation.org,
akpm@...ux-foundation.org, vbabka@...e.cz, dhowells@...hat.com,
surenb@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
# see patch description <stable@...r.kernel.org>,
Huang Ying <ying.huang@...el.com>,
Naoya Horiguchi <naoya.horiguchi@...ux.dev>
Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of
THP
On 27.02.24 15:52, Zi Yan wrote:
> On 27 Feb 2024, at 9:11, David Hildenbrand wrote:
>
>> On 14.02.24 15:34, David Hildenbrand wrote:
>>> On 14.02.24 15:18, Matthew Wilcox wrote:
>>>> On Wed, Feb 14, 2024 at 12:04:10PM +0530, Charan Teja Kalla wrote:
>>>>>> 1) Is it broken in 5.15? Did you actually try to reproduce or is this
>>>>>> just a guess?
>>>>>>
>>>>>
>>>>> We didn't run the tests with THP enabled on 5.15, __so we didn't
>>>>> encounter this issue__ on older to 6.1 kernels.
>>>>>
>>>>> I mentioned that issue exists is based on my understanding after code
>>>>> walk through. To be specific, I just looked to the
>>>>> migrate_pages()->..->migrate_page_move_mapping() &
>>>>> __split_huge_page_tail() where the ->private field of thp sub-pages is
>>>>> not filled with swap entry. If it could have set, I think these are the
>>>>> only places where it would have done, per my understanding. CMIW.
>>>>
>>>> I think you have a misunderstanding. David's patch cfeed8ffe55b (part
>>>> of 6.6) _stopped_ us using the tail ->private entries. So in 6.1, these
>>>> tail pages should already have page->private set, and I don't understand
>>>> what you're fixing.
>>>
>>> I think the issue is, that migrate_page_move_mapping() /
>>> folio_migrate_mapping() would update ->private for a folio in the
>>> swapcache (head page)
>>>
>>> newfolio->private = folio_get_private(folio);
>>>
>>> but not the ->private of the tail pages.
>>>
>>> So once you migrate a THP that is in the swapcache, ->private of the
>>> tail pages would not be migrated and, therefore, be stale/wrong.
>>>
>>> Even before your patch that was the case.
>>>
>>> Looking at migrate_page_move_mapping(), we had:
>>>
>>> if (PageSwapBacked(page)) {
>>> __SetPageSwapBacked(newpage);
>>> if (PageSwapCache(page)) {
>>> SetPageSwapCache(newpage);
>>> set_page_private(newpage, page_private(page));
>>> }
>>> } else {
>>> VM_BUG_ON_PAGE(PageSwapCache(page), page);
>>> }
>>>
>>>
>>> I don't immediately see where the tail pages would similarly get updated
>>> (via set_page_private).
>>>
>>> With my patch the problem is gone, because the tail page entries don't
>>> have to be migrated, because they are unused.
>>>
>>>
>>> Maybe this was an oversight from THP_SWAP -- 38d8b4e6bdc8 ("mm, THP,
>>> swap: delay splitting THP during swap out").
>>>
>>> It did update __add_to_swap_cache():
>>>
>>> for (i = 0; i < nr; i++) {
>>> set_page_private(page + i, entry.val + i);
>>> error = radix_tree_insert(&address_space->page_tree,
>>> idx + i, page + i);
>>> if (unlikely(error))
>>> break;
>>> }
>>>
>>> and similarly __delete_from_swap_cache().
>>>
>>> But I don't see any updates to migration code.
>>>
>>> Now, it could be that THP migration was added later (post 2017), in that
>>> case the introducing commit would not have been 38d8b4e6bdc8.
>>>
>>
>> Let's continue:
>>
>> The introducing commit is likely either
>>
>> (1) 38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out")
>>
>> That one added THP_SWAP, but THP migration wasn't supported yet AFAIKS.
>>
>> -> v4.13
>>
>> (2) 616b8371539a6 ("mm: thp: enable thp migration in generic path")
>
> I think this is the one, since it makes THP entering migrate_page_move_mapping()
> possible.
>
>>
>> Or likely any of the following that actually allocate THP for migration:
>>
>> 8135d8926c08e mm: memory_hotplug: memory hotremove supports thp migration
>> e8db67eb0ded3 mm: migrate: move_pages() supports thp migration
>> c8633798497ce mm: mempolicy: mbind and migrate_pages support thp migration
>>
>> That actually enable THP migration.
>>
>> -> v4.14
>>
>>
>> So likely we'd have to fix the stable kernels:
>>
>> 4.19
>> 5.4
>> 5.10
>> 5.15
>> 6.1
>>
>> That's a lot of pre-folio code. A backport of my series likely won't really make any sense.
>>
>> Staring at 4.19.307 code base, we likely have to perform a stable-only fix that properly handles the swapcache of compoud pages in migrate_page_move_mapping().
>
> Something like (applies to v4.19.307):
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 171573613c39..59878459c28c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -514,8 +514,13 @@ int migrate_page_move_mapping(struct address_space *mapping,
> if (PageSwapBacked(page)) {
> __SetPageSwapBacked(newpage);
> if (PageSwapCache(page)) {
> + int i;
> +
> SetPageSwapCache(newpage);
> - set_page_private(newpage, page_private(page));
> + for (i = 0; i < (1 << compound_order(page)); i++) {
> + set_page_private(newpage + i,
> + page_private(page + i));
> + }
> }
> } else {
> VM_BUG_ON_PAGE(PageSwapCache(page), page);
I'm wondering if there is a swapcache update missing as well.
--
Cheers,
David / dhildenb
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