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mm, percpu: do not consider sleepable allocations atomic

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[ Upstream commit 9a5b183941 ]

28307d938f ("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: 28307d938f ("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>
This commit is contained in:
Michal Hocko
2025-11-17 17:36:04 +08:00
committed by Greg Kroah-Hartman
parent c0a9c2c1b7
commit 34c93e96c3
1 changed files with 7 additions and 1 deletions
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8
mm/percpu.c
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@@ -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);
}
/**