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Patch series "mm/percpu: Fix possible NOFS/NOIO reclaim recursion", v4. Commit9a5b183941("mm, percpu: do not consider sleepable allocations atomic") allowed GFP_NOFS and GFP_NOIO percpu allocations to use pcpu_alloc_mutex and the chunk creation slow path. This restored the allocation capability that was lost when those constrained allocations were treated as atomic, but it also makes the percpu slow path visible to callers from constrained reclaim contexts. There are two related problems. First, the create and populate slow paths do not fully preserve the caller's allocation constraints. pcpu_alloc_noprof() derives pcpu_gfp from the caller supplied GFP mask and passes it down to the percpu backing page allocator. However, chunk creation calls pcpu_get_vm_areas(), and chunk population can allocate temporary metadata or vmalloc page tables while mapping backing pages. Those internal allocations can still use GFP_KERNEL, so a caller using GFP_NOFS or GFP_NOIO can enter unconstrained FS or IO reclaim while holding pcpu_alloc_mutex. One possible case is blk-cgroup after commit5d726c4dbe("blk-cgroup: fix possible deadlock while configuring policy"). blkg_conf_prep() now serializes against blkcg_deactivate_policy() with q->blkcg_mutex, and blkg_alloc() uses GFP_NOIO because queue freeze and IO reclaim dependencies can otherwise deadlock. If the percpu slow path loses that GFP_NOIO context, direct reclaim or writeback can issue IO to a frozen queue while q->blkcg_mutex is held. Second, allowing sleepable GFP_NOFS/GFP_NOIO allocations to take pcpu_alloc_mutex means that unconstrained backing allocations made under the mutex can create an FS/IO reclaim dependency against a constrained caller which already holds an FS or IO lock and then waits for pcpu_alloc_mutex. This series fixes those issues in three steps: - pass the caller supplied GFP mask into pcpu_get_vm_areas() and use it for vmalloc metadata and KASAN shadow allocations; - pass the GFP mask through the chunk population path, including the temporary pages array and vmalloc page table allocation scope; - restrict percpu backing allocations performed while holding pcpu_alloc_mutex to GFP_NOIO, so they cannot recurse into IO or FS reclaim. This keeps sleepable GFP_NOFS/GFP_NOIO percpu allocations working, while avoiding the reclaim recursion risks introduced by making those allocations eligible for the mutex-protected slow path. This patch (of 4): pcpu_alloc_noprof() derives pcpu_gfp from the caller supplied GFP mask and passes it down to the backing percpu allocator. However, when the percpu vmalloc allocator has to create a new chunk, pcpu_create_chunk() calls pcpu_get_vm_areas() to allocate the corresponding vmalloc areas. pcpu_get_vm_areas() currently performs its internal allocations with GFP_KERNEL, including vmap area metadata, vm_struct metadata and KASAN vmalloc shadow population. This means that a caller which deliberately uses GFP_NOFS or GFP_NOIO can still enter FS or IO reclaim while creating the vmalloc areas for a new percpu chunk. One possible case is blk-cgroup after commit5d726c4dbe("blk-cgroup: fix possible deadlock while configuring policy"). blkg_conf_prep() now serializes against blkcg_deactivate_policy() with q->blkcg_mutex, and blkg_alloc() was changed to GFP_NOIO for that reason: CPU0: blkg_conf_prep() mutex_lock(q->blkcg_mutex) blkg_alloc(..., GFP_NOIO) alloc_percpu_gfp(..., GFP_NOIO) pcpu_alloc_noprof(..., GFP_NOIO) pcpu_create_chunk(GFP_NOIO) pcpu_get_vm_areas() -> if percpu chunks are exhausted, chunk create may do internal GFP_KERNEL allocations -> direct reclaim / writeback can issue IO to this queue -> IO waits because the queue is frozen CPU1: blkcg_deactivate_policy() blk_mq_freeze_queue(q) mutex_lock(q->blkcg_mutex) -> waits for CPU0 ... unfreeze only happens after q->blkcg_mutex is acquired/released So the concern is that the caller deliberately uses GFP_NOIO because it may hold a lock which can be acquired after queue freeze, but the percpu slow path can temporarily lose that allocation context. Pass the caller supplied GFP mask from pcpu_create_chunk() to pcpu_get_vm_areas(), and use it for the internal vmalloc metadata and KASAN shadow allocations. Link: https://lore.kernel.org/20260618130414.96383-1-kaitao.cheng@linux.dev Link: https://lore.kernel.org/20260618130414.96383-2-kaitao.cheng@linux.dev Fixes:9a5b183941("mm, percpu: do not consider sleepable allocations atomic") Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com> Reviewed-by: Shivam Kalra <shivamkalra98@zohomail.in> Acked-by: Dennis Zhou <dennis@kernel.org> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Christoph Lameter <cl@gentwo.org> Cc: Pedro Falcato <pfalcato@suse.de> Cc: Tejun Heo <tj@kernel.org> Cc: Vlastimil Babka <vbabka@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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