linux-kernel - [PATCH 6.12.y] mm, percpu: do not consider sleepable allocations atomic

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Message-ID: <20251117093604.551707-1-mambaxin@163.com>
Date: Mon, 17 Nov 2025 17:36:04 +0800
From: mambaxin@....com
To: dennis@...nel.org,
	tj@...nel.org,
	cl@...ux.com,
	akpm@...ux-foundation.org,
	gregkh@...uxfoundation.org,
	mhocko@...e.com
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org,
	Vlastimil Babka <vbabka@...e.cz>,
	Filipe David Manana <fdmanana@...e.com>,
	chenxin <chenxinxin@...omi.com>
Subject: [PATCH 6.12.y] mm, percpu: do not consider sleepable allocations atomic

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

[ Upstream commit 9a5b183941b52f84c0f9e5f27ce44e99318c9e0f ]

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 itself 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.

Link: https://lkml.kernel.org/r/20250206122633.167896-1-mhocko@kernel.org
Fixes: 28307d938fb2 ("percpu: make pcpu_alloc() aware of current gfp context")
Signed-off-by: Michal Hocko <mhocko@...e.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Cc: Dennis Zhou <dennis@...nel.org>
Cc: Filipe David Manana <fdmanana@...e.com>
Cc: Tejun Heo <tj@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: chenxin <chenxinxin@...omi.com>
---
 mm/percpu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index fb0307723da6..44764720b6d8 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);

 	/*
@@ -2203,7 +2203,12 @@ 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);

@@ -2214,6 +2219,7 @@ static void pcpu_balance_workfn(struct work_struct *work)

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

 /**
-- 
2.50.1