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
