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 <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>
---
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)
--
2.35.3
On Sat, Oct 22, 2022 at 12:51:06AM -0700, Hugh Dickins wrote:
> 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.
>
The intent behind dumping the head was because I expected we'd be mid-split
and the head page was more meaningful but your patch also works.
> 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:
This was also on purpose. I suspected the warning would create a few new
reports but if page->private should be 0 then it's better to start catching
it now instead of later. At least we're still in an early rc kernel.
> 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 <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Matthew Wilcox (Oracle) <[email protected]>
> Cc: <[email protected]>
Thanks Hugh.
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs