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spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 24, 2025 at 09:43:45AM +0200, Alexei Starovoitov wrote: > On Wed, Sep 24, 2025 at 2:41 AM Harry Yoo wrote: > > > > On Mon, Sep 08, 2025 at 06:00:07PM -0700, Alexei Starovoitov wrote: > > > From: Alexei Starovoitov > > > > > > kmalloc_nolock() relies on ability of local_trylock_t to detect > > > the situation when per-cpu kmem_cache is locked. > > > > > > In !PREEMPT_RT local_(try)lock_irqsave(&s->cpu_slab->lock, flags) > > > disables IRQs and marks s->cpu_slab->lock as acquired. > > > local_lock_is_locked(&s->cpu_slab->lock) returns true when > > > slab is in the middle of manipulating per-cpu cache > > > of that specific kmem_cache. > > > > > > kmalloc_nolock() can be called from any context and can re-enter > > > into ___slab_alloc(): > > > kmalloc() -> ___slab_alloc(cache_A) -> irqsave -> NMI -> bpf -> > > > kmalloc_nolock() -> ___slab_alloc(cache_B) > > > or > > > kmalloc() -> ___slab_alloc(cache_A) -> irqsave -> tracepoint/kprobe -> bpf -> > > > kmalloc_nolock() -> ___slab_alloc(cache_B) > > > > > > Hence the caller of ___slab_alloc() checks if &s->cpu_slab->lock > > > can be acquired without a deadlock before invoking the function. > > > If that specific per-cpu kmem_cache is busy the kmalloc_nolock() > > > retries in a different kmalloc bucket. The second attempt will > > > likely succeed, since this cpu locked different kmem_cache. > > > > > > Similarly, in PREEMPT_RT local_lock_is_locked() returns true when > > > per-cpu rt_spin_lock is locked by current _task_. In this case > > > re-entrance into the same kmalloc bucket is unsafe, and > > > kmalloc_nolock() tries a different bucket that is most likely is > > > not locked by the current task. Though it may be locked by a > > > different task it's safe to rt_spin_lock() and sleep on it. > > > > > > Similar to alloc_pages_nolock() the kmalloc_nolock() returns NULL > > > immediately if called from hard irq or NMI in PREEMPT_RT. > > > > > > kfree_nolock() defers freeing to irq_work when local_lock_is_locked() > > > and (in_nmi() or in PREEMPT_RT). > > > > > > SLUB_TINY config doesn't use local_lock_is_locked() and relies on > > > spin_trylock_irqsave(&n->list_lock) to allocate, > > > while kfree_nolock() always defers to irq_work. > > > > > > Note, kfree_nolock() must be called _only_ for objects allocated > > > with kmalloc_nolock(). Debug checks (like kmemleak and kfence) > > > were skipped on allocation, hence obj = kmalloc(); kfree_nolock(obj); > > > will miss kmemleak/kfence book keeping and will cause false positives. > > > large_kmalloc is not supported by either kmalloc_nolock() > > > or kfree_nolock(). > > > > > > Signed-off-by: Alexei Starovoitov > > > --- > > > > On the up-to-date version [1] of this patch, > > I tried my best to find flaws in the code, but came up empty this time. > > Here's hoping :) > Much appreciate all the feedback and reviews during > this long journey (v1 was back in April). Thanks for pushing this forward and incorporating feedback so far. Cheers! And hopefully nothing serious comes up 🙏 -- Cheers, Harry / Hyeonggon