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

[Zi Yan]

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);

for all stable kernels above?

--
Best Regards,
Yan, Zi

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