+ mm-fix-is_pinnable_page-against-on-cma-page.patch added to mm-unstable branch

* + mm-fix-is_pinnable_page-against-on-cma-page.patch added to mm-unstable branch
@ 2022-05-14  0:18 Andrew Morton
  0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2022-05-14  0:18 UTC (permalink / raw)
  To: mm-commits, sfr, paulmck, joaodias, jhubbard, david, minchan, akpm


The patch titled
     Subject: mm: fix is_pinnable_page against on cma page
has been added to the -mm mm-unstable branch.  Its filename is
     mm-fix-is_pinnable_page-against-on-cma-page.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-fix-is_pinnable_page-against-on-cma-page.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Minchan Kim <minchan@kernel.org>
Subject: mm: fix is_pinnable_page against on cma page

Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA so
current is_pinnable_page could miss CMA pages which has MIGRATE_ ISOLATE. 
It ends up pinning CMA pages as longterm at pin_user_pages APIs so CMA
allocation keep failed until the pin is released.

     CPU 0                                   CPU 1 - Task B

cma_alloc
alloc_contig_range
                                        pin_user_pages_fast(FOLL_LONGTERM)
change pageblock as MIGRATE_ISOLATE
                                        internal_get_user_pages_fast
                                        lockless_pages_from_mm
                                        gup_pte_range
                                        try_grab_folio
                                        is_pinnable_page
                                          return true;
                                        So, pinned the page successfully.
page migration failure with pinned page
                                        ..
                                        .. After 30 sec
                                        unpin_user_page(page)

CMA allocation succeeded after 30 sec.

The CMA allocation path protects the migration type change race using
zone->lock but what GUP path need to know is just whether the page is on
CMA area or not rather than exact migration type.  Thus, we don't need
zone->lock but just checks migration type in either of (MIGRATE_ISOLATE
and MIGRATE_CMA).

Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause rejecting
of pinning pages on MIGRATE_ISOLATE pageblocks even though it's neither
CMA nor movable zone if the page is temporarily unmovable.  However, such
a migration failure by unexpected temporal refcount holding is general
issue, not only come from MIGRATE_ISOLATE and the MIGRATE_ISOLATE is also
transient state like other temporal elevated refcount problem.

[minchan@kernel.org: restore __mt definition]
[minchan@kernel.org: changes per Paul and John]
  Link: https://lkml.kernel.org/r/20220512204143.3961150-1-minchan@kernel.org
Link: https://lkml.kernel.org/r/20220510211743.95831-1-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: John Dias <joaodias@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h~mm-fix-is_pinnable_page-against-on-cma-page
+++ a/include/linux/mm.h
@@ -1625,8 +1625,21 @@ static inline bool page_needs_cow_for_dm
 #ifdef CONFIG_MIGRATION
 static inline bool is_pinnable_page(struct page *page)
 {
-	return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
-		is_zero_pfn(page_to_pfn(page));
+#ifdef CONFIG_CMA
+	/*
+	 * Defend against future compiler LTO features, or code refactoring
+	 * that inlines the above function, by forcing a single read. Because,
+	 * this routine races with set_pageblock_migratetype(), and we want to
+	 * avoid reading zero, when actually one or the other flags was set.
+	 */
+	int __mt = get_pageblock_migratetype(page);
+	int mt = __READ_ONCE(__mt);
+
+	if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
+		return false;
+#endif
+
+	return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
 }
 #else
 static inline bool is_pinnable_page(struct page *page)
_

Patches currently in -mm which might be from minchan@kernel.org are

mm-dont-be-stuck-to-rmap-lock-on-reclaim-path.patch
mm-fix-is_pinnable_page-against-on-cma-page.patch


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