Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP

[Zi Yan <ziy@nvidia.com>]

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On 27 Feb 2024, at 10:20, Zi Yan wrote:

> On 27 Feb 2024, at 10:01, David Hildenbrand wrote:
>
>> 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.
>
> It seems that e71769ae5260 ("mm: enable thp migration for shmem thp") fixed
> swapcache entry part, starting from v4.19.

For v6.1, the fix would like below?

diff --git a/mm/migrate.c b/mm/migrate.c
index c93dd6a31c31..c5968021fde0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -423,8 +423,12 @@ int folio_migrate_mapping(struct address_space *mapping,
        if (folio_test_swapbacked(folio)) {
                __folio_set_swapbacked(newfolio);
                if (folio_test_swapcache(folio)) {
+                       int i;
+
                        folio_set_swapcache(newfolio);
-                       newfolio->private = folio_get_private(folio);
+                       for (i = 0; i < nr; i++)
+                               set_page_private(folio_page(newfolio, i),
+                                       page_private(folio_page(folio, i)));
                }
                entries = nr;
        } else {

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Best Regards,
Yan, Zi

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