Re: mm/huge_memory: do not clobber swp_entry_t during THP split

Tvrtko Ursulin
2022-10-25T08:50:21Z

On 24/10/2022 15:23, Mel Gorman wrote:
> On Mon, Oct 24, 2022 at 02:04:50PM +0100, Tvrtko Ursulin wrote:
>>
>> Hi Mel, mm experts,
>>
>> With 6.1-rc2 we started hitting the WARN_ON added in 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t during THP split") in i915 automated CI:
>>
>
> Thanks for the report. As shmem pages pages are allocated via vma_alloc_folio
> and are compound pages, can you try the following patch please? If it
> still triggers, please post the new oops as it'll include the tail page
> information.
>
> --8<--
> From: Hugh Dickins <hughd@google.com>
> Subject: [PATCH] mm: prep_compound_tail() clear page->private
>
> Although page allocation always clears page->private in the first page
> or head page of an allocation, it has never made a point of clearing
> page->private in the tails (though 0 is often what is already there).
>
> But now commit 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t
> during THP split") issues a warning when page_tail->private is found to
> be non-0 (unless it's swapcache).
>
> Change that warning to dump page_tail (which also dumps head), instead
> of just the head: so far we have seen dead000000000122, dead000000000003,
> dead000000000001 or 0000000000000002 in the raw output for tail private.
>
> We could just delete the warning, but today's consensus appears to want
> page->private to be 0, unless there's a good reason for it to be set:
> so now clear it in prep_compound_tail() (more general than just for THP;
> but not for high order allocation, which makes no pass down the tails).
>
> Fixes: 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t during THP split")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: <stable@vger.kernel.org>
> ---
> mm/huge_memory.c | 2 +-
> mm/page_alloc.c | 1 +
> 2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 03fc7e5edf07..561a42567477 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2462,7 +2462,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
> * Fix up and warn once if private is unexpectedly set.
> */
> if (!folio_test_swapcache(page_folio(head))) {
> - VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head);
> + VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
> page_tail->private = 0;
> }
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b5a6c815ae28..218b28ee49ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -807,6 +807,7 @@ static void prep_compound_tail(struct page *head, int tail_idx)
>
> p->mapping = TAIL_MAPPING;
> set_compound_head(p, head);
> + set_page_private(p, 0);
> }
>
> void prep_compound_page(struct page *page, unsigned int order)

The patch seems to fix our CI runs. Is it considered final version? If
so I can temporarily put it in until it arrives via the next rc -
assuming that would be the flow from upstream pov?

Regards,

Tvrtko