linux-kernel - [PATCH] mm, percpu: do not consider sleepable allocations atomic

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Message-ID: <20250206122633.167896-1-mhocko@kernel.org>
Date: Thu,  6 Feb 2025 13:26:33 +0100
From: Michal Hocko <mhocko@...nel.org>
To: Dennis Zhou <dennis@...nel.org>,
	Tejun Heo <tj@...nel.org>,
	Filipe Manana <fdmanana@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	<linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: [PATCH] mm, percpu: do not consider sleepable allocations atomic

From: Michal Hocko <mhocko@...e.com>

28307d938fb2 ("percpu: make pcpu_alloc() aware of current gfp context")
has fixed a reclaim recursion for scoped GFP_NOFS context. It has done
that by avoiding taking pcpu_alloc_mutex. This is a correct solution as
the worker context with full GFP_KERNEL allocation/reclaim power and which
is using the same lock cannot block the NOFS pcpu_alloc caller.

On the other hand this is a very conservative approach that could lead
to failures because pcpu_alloc lockless implementation is quite limited.

We have a bug report about premature failures when scsi array of 193
devices is scanned. Sometimes (not consistently) the scanning aborts
because the iscsid daemon fails to create the queue for a random scsi
device during the scan. iscsid itslef is running with PR_SET_IO_FLUSHER
set so all allocations from this process context are GFP_NOIO. This in
turn makes any pcpu_alloc lockless (without pcpu_alloc_mutex) which
leads to pre-mature failures.

It has turned out that iscsid has worked around this by dropping
PR_SET_IO_FLUSHER (https://github.com/open-iscsi/open-iscsi/pull/382)
when scanning host. But we can do better in this case on the kernel side
and use pcpu_alloc_mutex for NOIO resp.  NOFS constrained allocation
scopes too.  We just need the WQ worker to never trigger IO/FS reclaim.
Achieve that by enforcing scoped GFP_NOIO for the whole execution of
pcpu_balance_workfn (this will imply NOFS constrain as well). This will
remove the dependency chain and preserve the full allocation power of
the pcpu_alloc call.

While at it make is_atomic really test for blockable allocations.

Fixes: 28307d938fb2 ("percpu: make pcpu_alloc() aware of current gfp context
Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/percpu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index d8dd31a2e407..192c2a8e901d 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1758,7 +1758,7 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
 	gfp = current_gfp_context(gfp);
 	/* whitelisted flags that can be passed to the backing allocators */
 	pcpu_gfp = gfp & (GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
-	is_atomic = (gfp & GFP_KERNEL) != GFP_KERNEL;
+	is_atomic = !gfpflags_allow_blocking(gfp);
 	do_warn = !(gfp & __GFP_NOWARN);

 	/*
@@ -2204,7 +2204,12 @@ static void pcpu_balance_workfn(struct work_struct *work)
 	 * to grow other chunks.  This then gives pcpu_reclaim_populated() time
 	 * to move fully free chunks to the active list to be freed if
 	 * appropriate.
+	 *
+	 * Enforce GFP_NOIO allocations because we have pcpu_alloc users
+	 * constrained to GFP_NOIO/NOFS contexts and they could form lock
+	 * dependency through pcpu_alloc_mutex
 	 */
+	unsigned int flags = memalloc_noio_save();
 	mutex_lock(&pcpu_alloc_mutex);
 	spin_lock_irq(&pcpu_lock);

@@ -2215,6 +2220,7 @@ static void pcpu_balance_workfn(struct work_struct *work)

 	spin_unlock_irq(&pcpu_lock);
 	mutex_unlock(&pcpu_alloc_mutex);
+	memalloc_noio_restore(flags);
 }

 /**
-- 
2.48.1