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Date: Tue, 27 Feb 2024 16:01:01 +0100
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Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of
 THP
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To: Zi Yan <ziy@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>,
 Charan Teja Kalla <quic_charante@quicinc.com>, gregkh@linuxfoundation.org,
 akpm@linux-foundation.org, vbabka@suse.cz, dhowells@redhat.com,
 surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
 # see patch description <stable@vger.kernel.org>,
 Huang Ying <ying.huang@intel.com>,
 Naoya Horiguchi <naoya.horiguchi@linux.dev>
References: <1707814102-22682-1-git-send-email-quic_charante@quicinc.com>
 <a683e199-ce8a-4534-a21e-65f2528415a6@redhat.com>
 <8620c1a0-e091-46e9-418a-db66e621b9c4@quicinc.com>
 <845ca78f-913b-4a92-8b40-ff772a7ad333@redhat.com>
 <bc1a5e36-1983-1a39-4d06-8062993a4ca4@quicinc.com>
 <ZczLoOqdpMJpkO5N@casper.infradead.org>
 <f2ad5918-7e36-4a7c-a619-c6807cfca5ec@redhat.com>
 <30ea073d-0ccf-46e1-954d-e22f5cbf69f7@redhat.com>
 <1ABD022A-35FC-4A6E-ADAD-36F3D745FB91@nvidia.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <1ABD022A-35FC-4A6E-ADAD-36F3D745FB91@nvidia.com>
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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