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[76.182.20.124]) by smtp.gmail.com with ESMTPSA id 00721157ae682-72151e8f522sm1970907b3.3.2025.08.27.07.54.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Aug 2025 07:54:50 -0700 (PDT) Date: Wed, 27 Aug 2025 10:54:49 -0400 From: Josef Bacik To: Mateusz Guzik Cc: linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org, kernel-team@fb.com, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, brauner@kernel.org, viro@zeniv.linux.org.uk, amir73il@gmail.com Subject: Re: [PATCH v2 03/54] fs: rework iput logic Message-ID: <20250827145449.GA2271493@perftesting> References: Precedence: bulk X-Mailing-List: linux-fsdevel@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: On Wed, Aug 27, 2025 at 04:18:55PM +0200, Mateusz Guzik wrote: > On Wed, Aug 27, 2025 at 02:58:51PM +0200, Mateusz Guzik wrote: > > On Tue, Aug 26, 2025 at 11:39:03AM -0400, Josef Bacik wrote: > > > Currently, if we are the last iput, and we have the I_DIRTY_TIME bit > > > set, we will grab a reference on the inode again and then mark it dirty > > > and then redo the put. This is to make sure we delay the time update > > > for as long as possible. > > > > > > We can rework this logic to simply dec i_count if it is not 1, and if it > > > is do the time update while still holding the i_count reference. > > > > > > Then we can replace the atomic_dec_and_lock with locking the ->i_lock > > > and doing atomic_dec_and_test, since we did the atomic_add_unless above. > > > > > > Signed-off-by: Josef Bacik > > > --- > > > fs/inode.c | 23 ++++++++++++++--------- > > > 1 file changed, 14 insertions(+), 9 deletions(-) > > > > > > diff --git a/fs/inode.c b/fs/inode.c > > > index a3673e1ed157..13e80b434323 100644 > > > --- a/fs/inode.c > > > +++ b/fs/inode.c > > > @@ -1911,16 +1911,21 @@ void iput(struct inode *inode) > > > if (!inode) > > > return; > > > BUG_ON(inode->i_state & I_CLEAR); > > > -retry: > > > - if (atomic_dec_and_lock(&inode->i_count, &inode->i_lock)) { > > > - if (inode->i_nlink && (inode->i_state & I_DIRTY_TIME)) { > > > - atomic_inc(&inode->i_count); > > > - spin_unlock(&inode->i_lock); > > > - trace_writeback_lazytime_iput(inode); > > > - mark_inode_dirty_sync(inode); > > > - goto retry; > > > - } > > > + > > > + if (atomic_add_unless(&inode->i_count, -1, 1)) > > > + return; > > > + > > > + if (inode->i_nlink && (inode->i_state & I_DIRTY_TIME)) { > > > + trace_writeback_lazytime_iput(inode); > > > + mark_inode_dirty_sync(inode); > > > + } > > > + > > > + spin_lock(&inode->i_lock); > > > + if (atomic_dec_and_test(&inode->i_count)) { > > > + /* iput_final() drops i_lock */ > > > iput_final(inode); > > > + } else { > > > + spin_unlock(&inode->i_lock); > > > } > > > } > > > EXPORT_SYMBOL(iput); > > > -- > > > 2.49.0 > > > > > > > This changes semantics though. > > > > In the stock kernel the I_DIRTY_TIME business is guaranteed to be sorted > > out before the call to iput_final(). > > > > In principle the flag may reappear after mark_inode_dirty_sync() returns > > and before the retried atomic_dec_and_lock succeeds, in which case it > > will get cleared again. > > > > With your change the flag is only handled once and should it reappear > > before you take the ->i_lock, it will stay there. > > > > I agree the stock handling is pretty crap though. > > > > Your change should test the flag again after taking the spin lock but > > before messing with the refcount and if need be unlock + retry. > > > > I would not hurt to assert in iput_final that the spin lock held and > > that this flag is not set. > > > > Here is my diff to your diff to illustrate + a cosmetic change, not even > > compile-tested: > > > > diff --git a/fs/inode.c b/fs/inode.c > > index 421e248b690f..a9ae0c790b5d 100644 > > --- a/fs/inode.c > > +++ b/fs/inode.c > > @@ -1911,7 +1911,7 @@ void iput(struct inode *inode) > > if (!inode) > > return; > > BUG_ON(inode->i_state & I_CLEAR); > > - > > +retry: > > if (atomic_add_unless(&inode->i_count, -1, 1)) > > return; > > > > @@ -1921,12 +1921,19 @@ void iput(struct inode *inode) > > } > > > > spin_lock(&inode->i_lock); > > + > > + if (inode->i_count == 1 && inode->i_nlink && (inode->i_state & I_DIRTY_TIME)) { > > + spin_unlock(&inode->i_lock); > > + goto retry; > > + } > > + > > if (atomic_dec_and_test(&inode->i_count)) { > > - /* iput_final() drops i_lock */ > > - iput_final(inode); > > - } else { > > spin_unlock(&inode->i_lock); > > + return; > > } > > + > > + /* iput_final() drops i_lock */ > > + iput_final(inode); > > } > > EXPORT_SYMBOL(iput); > > > > Sorry for spam, but the more I look at this the more fucky the entire > ordeal appears to me. > > Before I get to the crux, as a side note I did a quick check if atomics > for i_count make any sense to begin with and I think they do, here is a > sample output from a friend tracing the ref value on iput: > > bpftrace -e 'kprobe:iput /arg0 != 0/ { @[((struct inode *)arg0)->i_count.counter] = count(); }' > > @[5]: 66 > @[4]: 4625 > @[3]: 11086 > @[2]: 30937 > @[1]: 151785 > > ... so plenty of non-last refs after all. > > I completely agree the mandatory ref trip to handle I_DIRTY_TIME is lame > and needs to be addressed. > > But I'm uneasy about maintaining the invariant that iput_final() does > not see the flag if i_nlink != 0 and my proposal as pasted is dodgy af > on this front. > > While here some nits: > 1. it makes sense to try mere atomics just in case someone else messed > with the count between handling of the dirty flag and taking the spin lock > 2. according to my quick test with bpftrace the I_DIRTY_TIME flag is > seen way less frequently than i_nlink != 0, so it makes sense to swap > the order in which they are checked. Interested parties can try it out > with: > bpftrace -e 'kprobe:iput /arg0 != 0/ { @[((struct inode *)arg0)->i_nlink != 0, ((struct inode *)arg0)->i_state & (1 << 11)] = count(); }' > 3. touch up the iput_final() unlock comment > > All that said, how about something like the thing below as the final > routine building off of your change. I can't submit a proper patch and > can't even compile-test. I don't need any credit should this get > grabbed. Thanks for this Mateusz, you're right I completely changed the logic by not doing this under the i_lock. This update looks reasonable to me, thank you for the analysis and review! Josef