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b=QXznbcORwAbGrrVlo2x0+VNGFnYXsk4uwGmrL7rKo0DrOsKqkbD13eUe8lo6pEWrC ubCdl5NsC34rfAj0Y7/LYG6Pbrr73btn2pFc8HiXEiEkU68h3bNQxBLE6ql6jxM5FF +2xURsoMG2jkGdmcbAb08EaILXVCwYq4gGjxcd3o= Date: Mon, 3 Feb 2025 16:19:58 -0800 From: Andrew Morton To: Sergey Senozhatsky Cc: Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking Message-Id: <20250203161958.b529e20fb7aa07645ed029eb@linux-foundation.org> In-Reply-To: References: <20250131090658.3386285-1-senozhatsky@chromium.org> <20250131090658.3386285-2-senozhatsky@chromium.org> <20250131145536.d7a0483331b0ebfde80a5754@linux-foundation.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 52D5640005 X-Stat-Signature: tf5bkrccfj5zjiqnp18obqi5jmpgwshm X-Rspamd-Server: rspam08 X-Rspam-User: X-HE-Tag: 1738628400-513429 X-HE-Meta: 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 xVK/lClc TYSJvMTvwUMQUQTuCGZEdmL/gaVHN5Z383HME2slb7WFn7Qjd13Us6OY8J71RhxJtykZ4eTRTz6WO0Lei3HXT4fkR+n5CMnSqBNFE2bTqw/fqy1OS+U2jkDg5TNPdiM7jZwjzlB64nb293+hGKgwqwE4VveO0x0BQnLSa0rhH2mWD44H/bl2yu/klkedMQEeaeNZlfI1joWqlR8NWHC/TDNrahpbYGjNT8c2Yc/4IzVYjMzX0kyAeL967LW0liF1THFRnBGRYX46yhBaPojhLdAIv1OkmuGUHvC5iV72qN23Asl2c/mP0qKeMnqBUk24jaXha+vY3W2H/uYdpMg8PfD7f/VmHX0kGJFj05dTJAiI9maCUUGTCCppHSihDVm0X5GnVGUnCbqv2TP1IFMmDlFi99TNHuZlmBhNMbMGYwVTBwFNmsrSuUk414Vv8U4lRQ7yJ61JOUa2nd+CPyrYGy9BVJfjyB0z7JZLRPUT3pGkgkPVQ7JB48C4NsGzs1yRLtc6DBIIuDhSa47bI/u8c3Omqt+ZvYoS75MvG26JKo6kWRWWMdnoxH4lqcMLz3vJS4HLW7oGdE+gxRzLdymBZDVEMpARITG5rjmhmp1PAP9/RzNJbHk6Xs7Qsoc6oNS6kWma+ X-Bogosity: Ham, tests=bogofilter, 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 Mon, 3 Feb 2025 12:26:12 +0900 Sergey Senozhatsky wrote: > On (25/01/31 14:55), Andrew Morton wrote: > > > +static void zram_slot_write_lock(struct zram *zram, u32 index) > > > +{ > > > + atomic_t *lock = &zram->table[index].lock; > > > + int old = atomic_read(lock); > > > + > > > + do { > > > + if (old != ZRAM_ENTRY_UNLOCKED) { > > > + cond_resched(); > > > + old = atomic_read(lock); > > > + continue; > > > + } > > > + } while (!atomic_try_cmpxchg(lock, &old, ZRAM_ENTRY_WRLOCKED)); > > > +} > > > > I expect that if the calling userspace process has realtime policy (eg > > SCHED_FIFO) then the cond_resched() won't schedule SCHED_NORMAL tasks > > and this becomes a busy loop. And if the machine is single-CPU, the > > loop is infinite. > > So for that scenario to happen zram needs to see two writes() to the same > index (page) simultaneously? Or read() and write() on the same index (page) > concurrently? Well, my point is that in the contended case, this "lock" operation can get stuck forever. If there are no contended cases, we don't need a lock! And I don't see how disabling the feature if PREEMPT=y will avoid this situation. cond_resched() won't schedule away from a realtime task to a non-realtime one - a policy which isn't related to preemption.