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b=e6s4hY1lG4WND/DAKwuOfxj8htQX4HgmaTsBAaTzgUfGJbpig5y7TRGrGiUErYdM+ R+y/cl2WHFhYKeJQDxKq8pMvqckrlFQbGXp67TQE0io2loutvcQPKdYh4JMWw0LqDf ikKY6n+Le4rmVFfBZbsFC3WfVvJtxZKbgbIv+VRnT1uhIvUAUqf6s6eNtu9KV1ATkK DVZ5ifyVlYj8c03hU7D+jzGsIPxZJSqipM8YbO7hC4zkWTkOkZZROcZLjQNzKnoeNm McZvz3pk38TcGF6DXmROyqAvD7WW4uSMjXlnqY2UUW+cN0vxjwHZLQgZ85nQy7eqI8 A1CWTFfgm0FpA== Message-ID: From: sashiko-bot@kernel.org Subject: Re: [PATCH v5 12/13] zram: borrow idle decompression streams in exclusive mode Reply-To: sashiko-reviews@lists.linux.dev To: "Sergey Senozhatsky" , "Andrew Morton" Cc: "Brian Geffon" , "Barry Song" , "Bo Zhang" , "Minchan Kim" , "Richard Chang" , linux-mm@kvack.org In-Reply-To: <20261018035125.3120704-13-senozhatsky@chromium.org> References: <20261018035125.3120704-1-senozhatsky@chromium.org> <20261018035125.3120704-13-senozhatsky@chromium.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Date: Mon, 19 Oct 2026 03:53:14 +0000 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: F1BE540002 X-Stat-Signature: hnnudzcywshfrzib5pxcs8cmonc1z1m3 X-Rspam-User: X-HE-Tag: 1792381995-6872 X-HE-Meta: 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 1QaYe5Im 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Thank you for your contribution! Sashiko AI review found 1 potential issue(= s) to consider: - [High] zram: raw_smp_processor_id() re-evaluated in zcomp_dstrm_excl_tryg= et() loop -- commit a4463f4c7972dc5e8129cff84e8c92b3498b7a73 Author: Sergey Senozhatsky zram: borrow idle decompression streams in exclusive mode This commit allows exclusive decompression stream requests to scan online CPUs and borrow an idle per-CPU decompression stream before falling back to sleeping on a per-zcomp waitqueue. > diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c > index 4fda13aac8965..c6d2fd23c6f66 100644 > --- a/drivers/block/zram/zcomp.c > +++ b/drivers/block/zram/zcomp.c > @@ -185,38 +185,67 @@ bool zcomp_shared_decomp(struct zcomp *comp) > return !comp->ops->create_dctx; > } > =20 > +static bool zcomp_dstrm_excl_trylock(struct zcomp_dstrm *zstrm) > +{ > + /* Cheap pre-check: don't touch a busy stream's cachelines */ > + if (mutex_is_locked(&zstrm->excl_lock)) > + return false; > + /* > + * Pin the stream. The write side of ->lock is only taken by > + * zcomp_cpu_dead(), which also sets ->local_copy to NULL. > + */ > + if (!down_read_trylock(&zstrm->lock)) > + return false; > + if (zstrm->local_copy && mutex_trylock(&zstrm->excl_lock)) > + return true; > + up_read(&zstrm->lock); > + return false; > +} > + > +static struct zcomp_dstrm *zcomp_dstrm_excl_tryget(struct zcomp *comp) > +{ > + struct zcomp_dstrm *zstrm; > + unsigned int cpu; > + > + /* Best-effort start CPU hint; migration during the scan is fine */ > + for_each_online_cpu_wrap(cpu, raw_smp_processor_id()) { > + zstrm =3D per_cpu_ptr(comp->dstream, cpu); > + if (zcomp_dstrm_excl_trylock(zstrm)) > + return zstrm; > + } > + return NULL; > +} [Severity: High] Does passing raw_smp_processor_id() directly as the start argument to for_each_online_cpu_wrap() in zcomp_dstrm_excl_tryget() break the loop termination logic when the task migrates during the scan? for_each_online_cpu_wrap() expands via for_each_cpu_wrap() to for_each_set_bit_wrap() in include/linux/find.h: #define for_each_set_bit_wrap(bit, addr, size, start) \ for ((bit) =3D find_next_bit_wrap((addr), (size), (start)); \ (bit) < (size); \ (bit) =3D __for_each_wrap((addr), (size), (start), (bit) + 1)) Because start is expanded inside __for_each_wrap() on every step, raw_smp_processor_id() is re-evaluated on each iteration rather than once at the beginning of the loop. __for_each_wrap() relies on start remaining constant across the loop to know whether the scan has wrapped around: /* If not wrapped around */ if (n > start) { /* and have a bit, just return it. */ bit =3D find_next_bit(bitmap, size, n); if (bit < size) return bit; /* Otherwise, wrap around and ... */ n =3D 0; } /* Search the other part. */ bit =3D find_next_bit(bitmap, start, n); return bit < start ? bit : size; Since zcomp_dstrm_get() calls zcomp_dstrm_excl_tryget() in preemptible context with migration enabled, if a task fails zcomp_dstrm_excl_trylock() on CPU k and migrates to CPU m >=3D k + 1 before the step expression runs, __for_each_wrap() sees n =3D k + 1 <=3D start (where start is now m). It th= en treats the scan as having already wrapped and only searches [k + 1, m) (returning size immediately if m =3D=3D k + 1), skipping CPU m and the remaining online CPUs. When that happens, zcomp_dstrm_excl_tryget() returns NULL and zcomp_dstrm_get() sleeps in wait_event() on comp->dstrm_waitq even when idle streams are available on the skipped CPUs. In the case where CPU k is concurrently taken offline (so zcomp_cpu_dead() sets local_copy to NULL without waking comp->dstrm_waitq and the task is migrated off CPU k) while all other online streams are idle, no task holds excl_lock to wake comp->dstrm_waitq and the caller can sleep in TASK_UNINTERRUPTIBLE indefinitely. Should raw_smp_processor_id() be stored in a local variable before passing it to for_each_online_cpu_wrap()? [ ... ] > --=20 Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20261018035125.3120= 704-1-senozhatsky@chromium.org?part=3D12