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Sridhar" Cc: akpm@linux-foundation.org, chengming.zhou@linux.dev, dsterba@suse.com, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, nphamcs@gmail.com, terrelln@fb.com, yosry@kernel.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, senozhatsky@chromium.org, herbert@gondor.apana.org.au, davem@davemloft.net, linux-crypto@vger.kernel.org, kernel-team@meta.com References: <20261013002140.28880-1-kanchanapsridhar2026@gmail.com> <20261103142600.279899-1-usama.arif@linux.dev> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 03/11/2026 22:05, Kanchana P. Sridhar wrote: > On Tue, Nov 3, 2026 at 6:26 AM Usama Arif wrote: >> >> On Mon, 12 Oct 2026 17:21:40 -0700 "Kanchana P. Sridhar" wrote: >> >>> On Fri, Oct 9, 2026 at 6:31 PM Usama Arif wrote: >>>> >>>> Stores and loads serialize on the same per-CPU acomp request and mutex. >>>> A low-priority store can be preempted as soon as the compressor drops >>>> its stream lock, while it still holds the mutex. A higher-priority load >>>> on that CPU then waits until the store runs again, which can take a >>>> long time when other tasks are runnable. >>>> >>>> Give compression and decompression their own request, completion wait >>>> and mutex. Since commit e2c3b6b21c77f ("mm: zswap: use SG list >>>> decompression APIs from zsmalloc"), the per-CPU buffer is only used for >>>> compression, so group it with the compression request. Independent >>>> requests can share the per-CPU transform; the codecs and drivers >>>> synchronize their shared state internally. Loads can still wait for >>>> each other on the decompression mutex, and stores still serialize on >>>> the compression mutex. >>>> >>>> acomp_request_free() ignores NULL requests, so drop the check in >>>> acomp_ctx_free(). >>>> >>>> This follows the proposal from Sergey Senozhatsky for the same split >>>> for zram [1]. >>>> >>>> [1] https://lore.kernel.org/all/20261005122036.718976-10-senozhatsky@chromium.org/ >>>> >>>> Signed-off-by: Usama Arif >>> >>> I have a question with regards to this patch. Is the intent to allow a >>> decompress thread to run concurrently with a store thread for a given process? >> >> The request is shared by tasks using the same CPU and zswap pool. >> These may be different processes or threads of the same process. >> Each operation still runs synchronously in its caller. >> >> The motivating case can happen entirely on one CPU: >> 1. Task A compresses page X and releases the codec's stream lock. >> 2. A is preempted while still holding the zswap request mutex. >> 3. Task B runs and faults in an already stored page Y. >> >> With the shared request, B blocks until A runs again and releases the >> mutex. Separate requests let B load Y while A's store is unfinished. >> The codec/driver still synchronizes its own shared state. >> >>> If so, I wonder if incurring context switch/PTE/TLB shootdowns performance >>> penalty is worth it? >> >> The switch to B already occurs with the existing code. This patch adds >> no worker threads, forced migration, PTE changes or TLB shootdowns. >> It avoids B's subsequent wait on A's request mutex. The 1-vCPU >> benchmark in the cover letter exercises this without CPU migration. >> >>> >>> With the proposed change, will zswap_store() continue to run on the "old" CPU >>> (i.e. the one to which the process got migrated), or is it a requirement that >>> the "old" process is in sleep state? If so, should we codify/document this >>> requirement? >>> >> >> There is no requirement for A to sleep. A can remain runnable while >> preempted. Its zswap_store() invocation resumes in the same task, on >> whichever CPU the scheduler selects. If it migrates after selecting >> the context, it retains that pointer and the corresponding mutex, >> as the existing code does. The same task cannot execute on two CPUs >> at once. >> >>> On the new CPU, is the same process "runnable", and if so, how do we ensure that >>> the "higher priority" decompress is handled while the zswap_store() prior to >>> preemption is still not done? >> >> Normal scheduler policy decides when B runs. Once it runs, the split >> removes its dependency on A releasing the shared request mutex. >> A load starting on another CPU already selects a different context >> today; migration is unnecessary for the benefit described here. >> >> Existing folio/swap-cache locking still orders accesses to the same >> page. The example above concerns two independent pages. > > Thanks for explaining the motivation for this change, and the detailed > analysis! Is it possible that this could cause compression contexts to > be starved of compute resources under memory pressure, potentially > causing deadlocks with page-faults in the worst-case scenario? The split removes an unnecessary dependency between unrelated pages. Under fair scheduling, runnable reclaimers still get CPU time, and a fault needing memory can perform direct reclaim itself. Faults on a page being stored still wait for its folio lock, so I don't see a new deadlock cycle here. > Since > compression is inherently slower than decompression, could this be > making it easier for swap page-faults to preempt compression. On non-PREEMPT_RT kernels, software compression already runs under the codec's stream spinlock with preemption disabled. The delay addressed here is after that lock is released; the split does not make active compression newly preemptible. > On a > related note, should direct reclaim (only) be allowed to use a > non-preemptible context via a spinlock? > I would keep the current split. The codec already protects its stream, while the outer context must remain sleepable for asynchronous completion waits. A spinlock cannot safely cover those waits.