mm: prep_compound_tail() clear page->private

Commit Message

Hugh Dickins Oct. 22, 2022, 7:51 a.m. UTC

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 <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: <[e-mail]>
---
 mm/huge_memory.c | 2 +-
 mm/page_alloc.c  | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

Comments

Mel Gorman Oct. 22, 2022, 7:34 p.m. UTC | #1

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 <[e-mail]>
> Cc: Mel Gorman <[e-mail]>
> Cc: Matthew Wilcox (Oracle) <[e-mail]>
> Cc: <[e-mail]>

Thanks Hugh.

Acked-by: Mel Gorman <[e-mail]>

Patch

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)