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

[David Hildenbrand <david@redhat.com>]

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")

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().

Ugly.

-- 
Cheers,

David / dhildenb