[PATCH v4 1/8] mm/page_alloc: reset can_retry_reserves when the slowpath restarts

[Gregory Price (Meta)]

Commit 2c4c3e29897d ("mm/page_alloc: simplify __alloc_pages_slowpath()
flow") retries an allocation once with reserve_flags when the caller
becomes eligible for reserves, guarded by can_retry_reserves.  The flag
is set at its declaration and never reset.

A restart after a cpuset update or a zonelist rebuild drops
reserve_flags and comes back with can_retry_reserves already false, so a
caller that cannot reclaim, such as GFP_NOWAIT | __GFP_MEMALLOC, fails
without trying the reserves again.

Reset can_retry_reserves at restart.  Only a cpuset or zonelist change
restarts the slowpath, so the retry still runs at most once per change.

With kprobes failing every attempt without reserves and the first with
them, and a memory block onlined during the walk, the allocation
returned NULL in 3 of 3 runs, and a page in 3 of 3 with this change.

Fixes: 2c4c3e29897d ("mm/page_alloc: simplify __alloc_pages_slowpath() flow")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
 mm/page_alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2daf358269ae..78569ac6f2d9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4783,7 +4783,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	unsigned int zonelist_iter_cookie;
 	int reserve_flags;
 	bool compact_first = false;
-	bool can_retry_reserves = true;
+	bool can_retry_reserves;
 	unsigned long alloc_start_time = jiffies;
 
 	/*
@@ -4817,6 +4817,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	no_progress_loops = 0;
 	compact_result = COMPACT_SKIPPED;
 	compact_priority = DEF_COMPACT_PRIORITY;
+	can_retry_reserves = true;
 	cpuset_mems_cookie = read_mems_allowed_begin();
 	zonelist_iter_cookie = zonelist_iter_begin();
 
-- 
2.56.0