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 --- 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); }

/**