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Thu, 03 Sep 2026 02:10:59 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 3 Sep 2026 11:10:57 +0200 To: Dev Jain Cc: Uladzislau Rezki , Andrew Morton , Ye Liu , Ye Liu , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: vmalloc: fix vmap_purge_lock livelock under memory pressure Message-ID: References: <20260828091753.299295-1-ye.liu@linux.dev> <20260828110503.e1eff32a9b7df9a8b2ddd4d6@linux-foundation.org> <54bd749f-04f5-4665-bd5f-d485e7197132@arm.com> <2e9488bb-feb6-4994-92e3-e5ef0c1ff5ec@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Sep 02, 2026 at 10:00:57AM +0530, Dev Jain wrote: > > > On 01/09/26 10:29 pm, Uladzislau Rezki wrote: > > On Tue, Sep 01, 2026 at 06:43:43PM +0200, Uladzislau Rezki wrote: > >> On Tue, Sep 01, 2026 at 05:07:30PM +0800, Ye Liu wrote: > >>> > >>> > >>> 在 2026/9/1 14:22, Dev Jain 写道: > >>>> > >>>> > >>>> On 31/08/26 3:24 pm, Uladzislau Rezki wrote: > >>>>> On Mon, Aug 31, 2026 at 11:39:14AM +0530, Dev Jain wrote: > >>>>>> > >>>>>> > >>>>>> On 28/08/26 11:35 pm, Andrew Morton wrote: > >>>>>>> On Fri, 28 Aug 2026 17:17:53 +0800 Ye Liu wrote: > >>>>>>> > >>>>>>>> From: Ye Liu > >>>>>>>> > >>>>>>>> The vmap_purge_lock mutex can be held for an extended period by > >>>>>>>> __purge_vmap_area_lazy() which calls flush_work() to wait for > >>>>>>>> purge_vmap_node workers while holding the lock. Under memory > >>>>>>>> pressure, those workers may themselves be blocked in direct > >>>>>>>> reclaim trying to acquire the same lock via the > >>>>>>>> vmap_node_shrink_scan() shrinker callback, creating a circular > >>>>>>>> dependency that deadlocks the entire system. > >>>>>>>> > >>>>>>>> Two places acquire vmap_purge_lock from paths that can be reached > >>>>>>>> during direct reclaim: > >>>>>>>> > >>>>>>>> 1. vmap_node_shrink_scan(): replace blocking guard(mutex) with > >>>>>>>> mutex_trylock(). This is a shrinker that only decays the vmap > >>>>>>>> pool and returns SHRINK_STOP without freeing memory; skipping a > >>>>>>>> decay cycle when the lock is contended is harmless and prevents > >>>>>>>> tasks from piling up on the mutex in the direct reclaim path. > >>>>>>>> > >>>>>>>> 2. reclaim_and_purge_vmap_areas(): replace mutex_lock() with > >>>>>>>> mutex_trylock(). This is called from the vmalloc allocation > >>>>>>>> overflow path; if trylock fails, another thread is already > >>>>>>>> purging and the allocator's retry will find freed space. The > >>>>>>>> notifier chain provides a fallback if the retry still fails. > >>>>>>>> > >>>>>>>> Both trylock failures break the circular dependency: the lock > >>>>>>>> holder's flush_work() can complete because workers are no longer > >>>>>>>> blocked on vmap_purge_lock in the direct reclaim path. > >>>>>>> > >>>>>>> Thanks. AI review expressed a couple of concerns: > >>>>>>> https://sashiko.dev/#/patchset/20260828091753.299295-1-ye.liu@linux.dev > >>>>>> > >>>>>> > >>>>>> Sounds legit to me. Now there is no guarantee of purge being successful, and we > >>>>>> can get a spurious failure. > >>>>>> > >>>>>> How about using a WQ_RECLAIM workqueue: > >>>>>> > >>>>>> vmap_purge_wq = alloc_workqueue("vmap_purge", > >>>>>> WQ_MEM_RECLAIM | WQ_PERCPU, 0); > >>>>>> > >>>>>> I see the same pattern in __lru_add_drain_all() and kmem_cache_init_late(). > >>>>>> > >>>>> WQ_MEM_RECLAIM makes sense but this is another patch. > >>>>> > >>>>> I copied here AI comment: > >>>>>> > >>>>>> Does replacing this blocking lock with a trylock break synchronization for > >>>>>> the callers? > >>>>>> > >>>>> No it does not. If someone is doing reclaim we do not wait and do not try > >>>>> to do it again thus fail allocation. > >>>>> > >>>>>> When vmalloc space is exhausted, alloc_vmap_area() calls > >>>>>> reclaim_and_purge_vmap_areas() and relies on its blocking behavior to ensure > >>>>>> that free space has actually been reclaimed before looping back to retry: > >>>>>> mm/vmalloc.c:alloc_vmap_area() { > >>>>>> ... > >>>>>> overflow: > >>>>>> if (!purged) { > >>>>>> reclaim_and_purge_vmap_areas(); > >>>>>> purged = 1; > >>>>>> goto retry; > >>>>>> } > >>>>>> ... > >>>>>> } > >>>>>> With this patch, if another thread holds vmap_purge_lock, mutex_trylock() > >>>>>> fails and the function returns immediately. > >>>>>> > >>>>> If reclaim is in progress and trylock fails a caller repeats only one > >>>>> time to retry an allocation. There is no any infinite loop. > >>>>> > >>>>>> > >>>>>> The allocator then retries > >>>>>> instantly without waiting for the concurrent purge to complete. > >>>>>> Because the retry fails and purged is already 1, could this cause the > >>>>>> allocation to abort and return a spurious vmalloc allocation failure > >>>>>> (-EBUSY or -ENOMEM)? > >>>>>> > >>>>> vmap space can be fragmented and not avail for 32-bit systems. For > >>>>> 64-bit system it is likely impossible. > >>>>> > >>>>> But, i think we can overt mutex_lock() into mutex_trylock() just only > >>>>> in the: > >>>>> > >>>>> static unsigned long > >>>>> vmap_node_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) > >>>>> { > >>>>> struct vmap_node *vn; > >>>>> > >>>>> guard(mutex)(&vmap_purge_lock); > >>>>> for_each_vmap_node(vn) > >>>>> decay_va_pool_node(vn, true); > >>>>> > >>>>> return SHRINK_STOP; > >>>>> } > >>>>> > >>>>> so the reclaim path is not blocked. It should also address an issue > >>>>> reported by the Ye Liu . > >>>> > >>>> IIUC you are suggesting mutex_trylock() only in the shrinker path. But > >>>> then, the following is possible no: take purge lock, try to get a > >>>> worker thread, worker thread is stuck in vmalloc -> alloc_vmap_area > >>>> -> reclaim_and_purge_vmap_areas -> take purge lock? > >>>> > >>>> > >>> > >>> Personally, I lean toward the WQ_MEM_RECLAIM workqueue solution. > >>> After taking a closer look at the code, I noticed a subtle but > >>> potentially problematic scenario: > >>> > >>> When drain_vmap_area_work acquires vmap_purge_lock and calls into > >>> __purge_vmap_area_lazy, it may subsequently invoke queue_work/queue_work_on > >>> on the same CPU's system_wq. If the newly queued work ends up waiting > >>> for an available worker on that same CPU, while the current worker is > >>> blocked waiting for that very work to complete (via flush_work), > >>> we could end up with a self-deadlock on a single CPU. > >>> > >>> Theoretically, this seems possible. I suspect the reason we don't > >>> see widespread reports of such deadlocks is that the nr_purge_helpers > >>> logic limits the number of asynchronous workers; when resources are tight, > >>> it falls back to synchronous execution (purge_vmap_node directly), > >>> which avoids queuing additional work. > >>> > >>> Using a dedicated workqueue with WQ_MEM_RECLAIM would provide a clean, > >>> explicit isolation—ensuring forward progress under memory pressure and > >>> eliminating the risk of interfering with other subsystems' workqueues. > >>> I believe this approach is more robust in the long run. > >>> > >>> Perhaps like the code below： > >>> the dedicated queue eliminates the self‑deadlock risk, and the trylock > >>> in the shrinker prevents recursive lock attempts from reclaim contexts. > >>> > >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c > >>> index bea9f76ed7e7..68fc1f5acb2f 100644 > >>> --- a/mm/vmalloc.c > >>> +++ b/mm/vmalloc.c > >>> @@ -2218,6 +2218,9 @@ static unsigned long lazy_max_pages(void) > >>> */ > >>> static DEFINE_MUTEX(vmap_purge_lock); > >>> > >>> +/* Workqueue for lazy vmap purging; WQ_MEM_RECLAIM guarantees progress. */ > >>> +static struct workqueue_struct *vmap_purge_wq; > >>> + > >>> /* for per-CPU blocks */ > >>> static void purge_fragmented_blocks_allcpus(void); > >>> > >>> @@ -2408,9 +2411,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end, > >>> INIT_WORK(&vn->purge_work, purge_vmap_node); > >>> > >>> if (cpumask_test_cpu(i, cpu_online_mask)) > >>> - schedule_work_on(i, &vn->purge_work); > >>> + queue_work_on(i, vmap_purge_wq, &vn->purge_work); > >>> else > >>> - schedule_work(&vn->purge_work); > >>> + queue_work(vmap_purge_wq, &vn->purge_work); > >>> > >>> nr_purge_helpers--; > >>> } else { > >>> @@ -5519,10 +5522,14 @@ vmap_node_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) > >>> { > >>> struct vmap_node *vn; > >>> > >>> - guard(mutex)(&vmap_purge_lock); > >>> + if (!mutex_trylock(&vmap_purge_lock)) > >>> + return SHRINK_STOP; > >>> + > >>> for_each_vmap_node(vn) > >>> decay_va_pool_node(vn, true); > >>> > >>> + mutex_unlock(&vmap_purge_lock); > >>> + > >>> return SHRINK_STOP; > >>> } > >>> > >>> @@ -5575,6 +5582,17 @@ void __init vmalloc_init(void) > >>> * Now we can initialize a free vmap space. > >>> */ > >>> vmap_init_free_space(); > >>> + > >>> + /* > >>> + * A dedicated workqueue for lazy vmap purging. WQ_MEM_RECLAIM > >>> + * reserves a rescue worker so queued purge work items are executed > >>> + * even under memory pressure, when workers of the system workqueue > >>> + * may be stuck in direct reclaim. > >>> + */ > >>> + vmap_purge_wq = alloc_workqueue("vmap_purge", > >>> + WQ_MEM_RECLAIM | WQ_PERCPU, 0); > >>> + WARN_ON(!vmap_purge_wq); > >>> + > >>> vmap_initialized = true; > >>> > >> I agree. We should have it and it should be as separate patch, i.e. > >> split vmap_node_shrink_scan() and dedicated per-cpu WQs per vmap drain. > >> > > And i sent out already the WQ_UNBOUND | WQ_MEM_RECLAIM and separate WQ > > for vmap drain logic. It looks like Andrew/me forgot about it: > > > > https://lore.kernel.org/all/20260331202352.879718-1-urezki@gmail.com/ > > Great! Perhaps resend it and we can review it? > I will and add you to Cc. -- Uladzislau Rezki