Patch "mm, percpu: do not consider sleepable allocations atomic" has been added to the 6.6-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    mm, percpu: do not consider sleepable allocations atomic

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mm-percpu-do-not-consider-sleepable-allocations-atomic.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From stable+bounces-194942-greg=kroah.com@vger.kernel.org Mon Nov 17 10:31:20 2025
From: mambaxin@163.com
Date: Mon, 17 Nov 2025 17:30:13 +0800
Subject: mm, percpu: do not consider sleepable allocations atomic
To: dennis@kernel.org, tj@kernel.org, cl@linux.com, akpm@linux-foundation.org, gregkh@linuxfoundation.org, mhocko@suse.com
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Vlastimil Babka <vbabka@suse.cz>, Filipe David Manana <fdmanana@suse.com>, chenxin <chenxinxin@xiaomi.com>
Message-ID: <20251117093013.545253-1-mambaxin@163.com>

From: Michal Hocko <mhocko@suse.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@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Filipe David Manana <fdmanana@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: chenxin <chenxinxin@xiaomi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/percpu.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1734,7 +1734,7 @@ static void __percpu *pcpu_alloc(size_t
 	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);
 
 	/*
@@ -2231,7 +2231,12 @@ static void pcpu_balance_workfn(struct w
 	 * 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);
 
@@ -2242,6 +2247,7 @@ static void pcpu_balance_workfn(struct w
 
 	spin_unlock_irq(&pcpu_lock);
 	mutex_unlock(&pcpu_alloc_mutex);
+	memalloc_noio_restore(flags);
 }
 
 /**


Patches currently in stable-queue which might be from mambaxin@163.com are

queue-6.6/mm-percpu-do-not-consider-sleepable-allocations-atomic.patch