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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 04/10] mm: remove set_page_count() from page_frag_alloc_align
Date: Wed, 17 Nov 2021 01:20:53 +0000	[thread overview]
Message-ID: <20211117012059.141450-5-pasha.tatashin@soleen.com> (raw)
In-Reply-To: <20211117012059.141450-1-pasha.tatashin@soleen.com>

set_page_count() unconditionally resets the value of _ref_count and that
is dangerous, as it is not programmatically verified. Instead we rely on
comments like: "OK, page count is 0, we can safely set it".

Add a new refcount function: page_ref_add_return() to return the new
refcount value after adding to it. Use the return value to verify that
the _ref_count was indeed the expected one.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 11 +++++++++++
 mm/page_alloc.c          |  6 ++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index f3c61dc6344a..27880aca2e2f 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -115,6 +115,17 @@ static inline void init_page_count(struct page *page)
 	set_page_count(page, 1);
 }
 
+static inline int page_ref_add_return(struct page *page, int nr)
+{
+	int old_val = atomic_fetch_add(nr, &page->_refcount);
+	int new_val = old_val + nr;
+
+	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
+	if (page_ref_tracepoint_active(page_ref_mod_and_return))
+		__page_ref_mod_and_return(page, nr, new_val);
+	return new_val;
+}
+
 static inline void page_ref_add(struct page *page, int nr)
 {
 	int old_val = atomic_fetch_add(nr, &page->_refcount);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c5952749ad40..e8e88111028a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5516,6 +5516,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 	unsigned int size = PAGE_SIZE;
 	struct page *page;
 	int offset;
+	int refcnt;
 
 	if (unlikely(!nc->va)) {
 refill:
@@ -5554,8 +5555,9 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 		/* if size can vary use size else just use PAGE_SIZE */
 		size = nc->size;
 #endif
-		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
+		/* page count is 0, set it to PAGE_FRAG_CACHE_MAX_SIZE + 1 */
+		refcnt = page_ref_add_return(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
+		VM_BUG_ON_PAGE(refcnt != PAGE_FRAG_CACHE_MAX_SIZE + 1, page);
 
 		/* reset page count bias and offset to start of new frag */
 		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
-- 
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 ` Pasha Tatashin [this message]
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 ` [RFC v2 09/10] mm: do not use atomic_set_release in page_ref_unfreeze() Pasha Tatashin
2021-11-17  1:20 ` [RFC v2 10/10] mm: use atomic_cmpxchg_acquire in page_ref_freeze() Pasha Tatashin

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