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

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Date: Wed, 14 Feb 2024 15:34:24 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>,
 Charan Teja Kalla <quic_charante@...cinc.com>
Cc: 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>
Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of
 THP

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.

-- 
Cheers,

David / dhildenb