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From: "Yosry Ahmed" Message-ID: <3f4ff25513fa8a6589288418694f614e4d0399c4@linux.dev> TLS-Required: No Subject: Re: [PATCH v5 12/18] zsmalloc: make zspage lock preemptible To: "Sergey Senozhatsky" Cc: "Sergey Senozhatsky" , "Andrew Morton" , "Kairui Song" , "Minchan Kim" , linux-mm@kvack.org, linux-kernel@vger.kernel.org In-Reply-To: References: <20250212063153.179231-1-senozhatsky@chromium.org> <20250212063153.179231-13-senozhatsky@chromium.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Stat-Signature: bpm63tikxqx6mt5pxrxojx6dymyadqiq X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: C068440007 X-HE-Tag: 1739410307-239397 X-HE-Meta: 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 a5H5fV78 F8fe8Q+za4TTOf//5XhMPTZRMu+TQ3qkeOzwceryPsHfhUD8ARbSD1iLxTyyauH+EfGi15XImcPHCVPJchuWMFVVzFyiUmZls1gW9Sk0hI+e6IMVJaxlqh6akd5dwuL3qk7z5MTqqiBm+LZKti/9kNW9POhccvRUbur6YJTVXEpbtyB+fWFTMXpsLsg== 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: >=20 >=20>=20 >=20> +{ > >=20 >=20> + atomic_t *lock =3D &zspage->lock; > >=20 >=20> + int old =3D ZS_PAGE_UNLOCKED; > >=20 >=20> + > >=20 >=20> + WARN_ON_ONCE(preemptible()); > >=20 >=20>=20=20 >=20>=20 >=20> Hmm I know I may have been the one suggesting this, but do we actu= ally > >=20 >=20> need it? We disable preemption explicitly anyway before holding th= e > >=20 >=20> lock. > >=20 >=20 > This is just to make sure that the precondition for >=20 >=20"writer is always atomic" is satisfied. But I can drop it. Right, but why do we care? Even if the context is not atomic, we disable = preemtion and make sure the context stays atomic throughout the lock crit= ical section. >=20 >=20>=20 >=20> size_class_lock(class); > >=20 >=20> - /* the migrate_write_lock protects zpage access via zs_map_objec= t */ > >=20 >=20> - migrate_write_lock(zspage); > >=20 >=20> + /* the zspage write_lock protects zpage access via zs_map_object= */ > >=20 >=20> + if (!zspage_try_write_lock(zspage)) { > >=20 >=20> + size_class_unlock(class); > >=20 >=20> + pool_write_unlock(pool); > >=20 >=20> + return -EINVAL; > >=20 >=20> + } > >=20 >=20> + > >=20 >=20> + /* We're committed, tell the world that this is a Zsmalloc page.= */ > >=20 >=20> + __zpdesc_set_zsmalloc(newzpdesc); > >=20 >=20>=20=20 >=20>=20 >=20> We used to do this earlier on, before any locks are held. Why is i= t > >=20 > > moved here? > >=20 >=20 > I want to do that only if zspaage write-trylock has succeeded (we didn'= t >=20 >=20have any error out paths before). Ack.