[PATCH] mm/percpu: avoid IO dependency in alloc_percpu

[Khazhismel Kumykov]

percpu_alloc users within noio context may block on other percpu_alloc
users doing IO due to the shared pcpu_alloc_mutex. pcpu_balance_workfn
already grabs under noio context - update remaining holders to also run
under noio.

Lockdep reported this as a circular locking dependency, with one caller
having entered fs_reclaim holding pcpu_alloc_mutex, and the second
caller (elevator init) grabbing pcpu_alloc_mutex in noio context.

Fixes: 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations atomic")

Signed-off-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
---
 mm/percpu.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index b0676b8054ed..50057403ae79 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1749,8 +1749,13 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
 	size_t bits, bit_align;
 
 	gfp = current_gfp_context(gfp);
-	/* whitelisted flags that can be passed to the backing allocators */
-	pcpu_gfp = gfp & (GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
+	/*
+	 * whitelisted flags that can be passed to the backing allocators.
+	 *
+	 * Allocations must be GFP_NOIO, else users in noio/nofs contexts may
+	 * form dependency through pcpu_alloc_mutex
+	 */
+	pcpu_gfp = gfp & (GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);
 	is_atomic = !gfpflags_allow_blocking(gfp);
 	do_warn = !(gfp & __GFP_NOWARN);
 
-- 
2.54.0.1189.g8c84645362-goog