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Wed, 12 Mar 2025 15:51:26 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id VcPOK36t0WenFAAAD6G6ig (envelope-from ); Wed, 12 Mar 2025 15:51:26 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 65CB5A0908; Wed, 12 Mar 2025 16:51:22 +0100 (CET) Date: Wed, 12 Mar 2025 16:51:22 +0100 From: Jan Kara To: Mateusz Guzik Cc: brauner@kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, audit@vger.kernel.org, axboe@kernel.dk Subject: Re: [PATCH] fs: dodge an atomic in putname if ref == 1 Message-ID: References: <20250311181804.1165758-1-mjguzik@gmail.com> Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250311181804.1165758-1-mjguzik@gmail.com> X-Rspamd-Queue-Id: BEDEB21195 X-Spam-Score: -4.01 X-Rspamd-Action: no action X-Spamd-Result: default: False [-4.01 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_RHS_NOT_FQDN(0.50)[]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FUZZY_BLOCKED(0.00)[rspamd.com]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; FREEMAIL_TO(0.00)[gmail.com]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; RCVD_TLS_LAST(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCPT_COUNT_SEVEN(0.00)[9]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.com:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO X-Spam-Level: On Tue 11-03-25 19:18:04, Mateusz Guzik wrote: > While the structure is refcounted, the only consumer incrementing it is > audit and even then the atomic operation is only needed when it > interacts with io_uring. > > If putname spots a count of 1, there is no legitimate way for anyone to > bump it. > > If audit is disabled, the count is guaranteed to be 1, which > consistently elides the atomic for all path lookups. If audit is > enabled, it still manages to elide the last decrement. > > Note the patch does not do anything to prevent audit from suffering > atomics. See [1] and [2] for a different approach. > > Benchmarked on Sapphire Rapids issuing access() (ops/s): > before: 5106246 > after: 5269678 (+3%) > > Link 1: https://lore.kernel.org/linux-fsdevel/20250307161155.760949-1-mjguzik@gmail.com/ > Link 2: https://lore.kernel.org/linux-fsdevel/20250307164216.GI2023217@ZenIV/ > Signed-off-by: Mateusz Guzik Yeah, I like this much more than the original. Feel free to add: Reviewed-by: Jan Kara Honza > This is an alternative to the patch I linked above. > > I think the improved commit message should also cover the feedback > Christian previously shared concerning it. > > This is a trivial win until the atomic issue gets resolved, I don't see > any reason to NOT include it. At the same time I don't have that much > interest arguing about it either. > > That is to say, here is a different take, if you don't like it, I'm > dropping the subject. > > cheers > > fs/namei.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/fs/namei.c b/fs/namei.c > index 06765d320e7e..add90981cfcd 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -275,14 +275,19 @@ EXPORT_SYMBOL(getname_kernel); > > void putname(struct filename *name) > { > + int refcnt; > + > if (IS_ERR_OR_NULL(name)) > return; > > - if (WARN_ON_ONCE(!atomic_read(&name->refcnt))) > - return; > + refcnt = atomic_read(&name->refcnt); > + if (refcnt != 1) { > + if (WARN_ON_ONCE(!refcnt)) > + return; > > - if (!atomic_dec_and_test(&name->refcnt)) > - return; > + if (!atomic_dec_and_test(&name->refcnt)) > + return; > + } > > if (name->name != name->iname) { > __putname(name->name); > -- > 2.43.0 > -- Jan Kara SUSE Labs, CR