The patch titled
Subject: mm, percpu: do not consider sleepable allocations atomic
has been added to the -mm mm-unstable branch. Its filename is
mm-percpu-do-not-consider-sleepable-allocations-atomic.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-percpu-do-not-consider-sleepable-allocations-atomic.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Michal Hocko <mhocko@xxxxxxxx>
Subject: mm, percpu: do not consider sleepable allocations atomic
Date: Thu, 6 Feb 2025 13:26:33 +0100
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.
Link: https://lkml.kernel.org/r/20250206122633.167896-1-mhocko@xxxxxxxxxx
Fixes: 28307d938fb2 ("percpu: make pcpu_alloc() aware of current gfp context
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Filipe David Manana <fdmanana@xxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/percpu.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
--- a/mm/percpu.c~mm-percpu-do-not-consider-sleepable-allocations-atomic
+++ a/mm/percpu.c
@@ -1745,7 +1745,7 @@ void __percpu *pcpu_alloc_noprof(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);
/*
@@ -2191,7 +2191,12 @@ 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);
@@ -2202,6 +2207,7 @@ static void pcpu_balance_workfn(struct w
spin_unlock_irq(&pcpu_lock);
mutex_unlock(&pcpu_alloc_mutex);
+ memalloc_noio_restore(flags);
}
/**
_
Patches currently in -mm which might be from mhocko@xxxxxxxx are
mm-percpu-do-not-consider-sleepable-allocations-atomic.patch
