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From: Pasha Tatashin <pasha.tatashin@soleen.com>
To: pasha.tatashin@soleen.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-m68k@lists.linux-m68k.org,
	anshuman.khandual@arm.com, willy@infradead.org,
	akpm@linux-foundation.org, william.kucharski@oracle.com,
	mike.kravetz@oracle.com, vbabka@suse.cz, geert@linux-m68k.org,
	schmitzmic@gmail.com, rostedt@goodmis.org, mingo@redhat.com,
	hannes@cmpxchg.org, guro@fb.com, songmuchun@bytedance.com,
	weixugc@google.com, gthelen@google.com, rientjes@google.com,
	pjt@google.com
Subject: [RFC v2 09/10] mm: do not use atomic_set_release in page_ref_unfreeze()
Date: Wed, 17 Nov 2021 01:20:58 +0000	[thread overview]
Message-ID: <20211117012059.141450-10-pasha.tatashin@soleen.com> (raw)
In-Reply-To: <20211117012059.141450-1-pasha.tatashin@soleen.com>

In we set the old _refcount value after verifying that the old value was
indeed 0.

VM_BUG_ON_PAGE(page_count(page) != 0, page);
< the _refcount may change here>
atomic_set_release(&page->_refcount, count);

To avoid the smal gap where _refcount may change lets verify the time
of the _refcount at the time of the set operation.

Use atomic_xchg_release() and at the set time verify that the value
was 0.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 8c76bf3bf7e1..26676d3bcd58 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -322,10 +322,9 @@ static inline int folio_ref_freeze(struct folio *folio, int count)
 
 static inline void page_ref_unfreeze(struct page *page, int count)
 {
-	VM_BUG_ON_PAGE(page_count(page) != 0, page);
-	VM_BUG_ON(count == 0);
+	int old_val = atomic_xchg_release(&page->_refcount, count);
 
-	atomic_set_release(&page->_refcount, count);
+	VM_BUG_ON_PAGE(count == 0 || old_val != 0, page);
 	if (page_ref_tracepoint_active(page_ref_unfreeze))
 		__page_ref_unfreeze(page, count);
 }
-- 
2.34.0.rc1.387.gb447b232ab-goog


  parent reply	other threads:[~2021-11-17  1:21 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-17  1:20 [RFC v2 00/10] Hardening page _refcount Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 01/10] mm: page_ref_add_unless() does not trace 'u' argument Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 02/10] mm: add overflow and underflow checks for page->_refcount Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 03/10] mm: Avoid using set_page_count() in set_page_recounted() Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 04/10] mm: remove set_page_count() from page_frag_alloc_align Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 05/10] mm: avoid using set_page_count() when pages are freed into allocator Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 06/10] mm: rename init_page_count() -> page_ref_init() Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 07/10] mm: remove set_page_count() Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 08/10] mm: simplify page_ref_* functions Pasha Tatashin
2021-11-17  1:20 ` Pasha Tatashin [this message]
2021-11-17  1:20 ` [RFC v2 10/10] mm: use atomic_cmpxchg_acquire in page_ref_freeze() Pasha Tatashin

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