[PATCH] mm/madvise: elevate page refcount while isolating in process_madvise()

[PATCH] mm/madvise: elevate page refcount while isolating in process_madvise()

[Charan Teja Reddy]

The documentation of isolate_lru_page() says that, "it must be called
with an elevated refcount on the page", which is not followed while
isolating pages in process_madvise() system call with advise
MADV_PAGEOUT. Fix it.

Signed-off-by: Charan Teja Reddy <quic_charante@quicinc.com>
---
 mm/madvise.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 0734db8..4c4a8e9 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -317,6 +317,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 	pte_t *orig_pte, *pte, ptent;
 	spinlock_t *ptl;
 	struct page *page = NULL;
+	int ret;
 	LIST_HEAD(page_list);
 
 	if (fatal_signal_pending(current))
@@ -373,12 +374,15 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		ClearPageReferenced(page);
 		test_and_clear_page_young(page);
 		if (pageout) {
-			if (!isolate_lru_page(page)) {
+			get_page(page);
+			ret = isolate_lru_page(page);
+			if (!ret) {
 				if (PageUnevictable(page))
 					putback_lru_page(page);
 				else
 					list_add(&page->lru, &page_list);
 			}
+			put_page(page);
 		} else
 			deactivate_page(page);
 huge_unlock:
@@ -459,12 +463,15 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		ClearPageReferenced(page);
 		test_and_clear_page_young(page);
 		if (pageout) {
-			if (!isolate_lru_page(page)) {
+			get_page(page);
+			ret = isolate_lru_page(page);
+			if (!ret) {
 				if (PageUnevictable(page))
 					putback_lru_page(page);
 				else
 					list_add(&page->lru, &page_list);
 			}
+			put_page(page);
 		} else
 			deactivate_page(page);
 	}
-- 
2.7.4

Re: [PATCH] mm/madvise: elevate page refcount while isolating in process_madvise()

[Matthew Wilcox]

On Wed, Dec 15, 2021 at 05:53:39PM +0530, Charan Teja Reddy wrote:
> The documentation of isolate_lru_page() says that, "it must be called
> with an elevated refcount on the page", which is not followed while
> isolating pages in process_madvise() system call with advise
> MADV_PAGEOUT. Fix it.

We hold the mmap_lock over the call to this function, so the reference
to the page from the page tables cannot go away.  There's no need to
grab an extra reference here.

Re: [PATCH] mm/madvise: elevate page refcount while isolating in process_madvise()

[Charan Teja Kalla]

On 12/15/2021 7:25 PM, Matthew Wilcox wrote:
> On Wed, Dec 15, 2021 at 05:53:39PM +0530, Charan Teja Reddy wrote:
>> The documentation of isolate_lru_page() says that, "it must be called
>> with an elevated refcount on the page", which is not followed while
>> isolating pages in process_madvise() system call with advise
>> MADV_PAGEOUT. Fix it.
> 
> We hold the mmap_lock over the call to this function, so the reference
> to the page from the page tables cannot go away.  There's no need to
> grab an extra reference here.

Thanks Matthew for clearing such fundamentals.

>