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

Michal Hocko
2025-02-06T12:26:46Z

From: Michal Hocko <mhocko@suse.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@suse.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