linux-kernel - Re: [PATCH] iomap: avoid unnecessary ifs_set_range

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Message-ID: <20250702120912.36380-1-alexjlzheng@tencent.com>
Date: Wed,  2 Jul 2025 20:09:12 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: djwong@...nel.org
Cc: alexjlzheng@...il.com,
	alexjlzheng@...cent.com,
	brauner@...nel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org
Subject: Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

On Tue, 1 Jul 2025 11:47:37 -0700, djwong@...nel.org wrote:
> On Tue, Jul 03, 2025 at 10:48:47PM +0800, alexjlzheng@...il.com wrote:
> > From: Jinliang Zheng <alexjlzheng@...cent.com>
> > 
> > In the buffer write path, iomap_set_range_uptodate() is called every
> > time iomap_end_write() is called. But if folio_test_uptodate() holds, we
> > know that all blocks in this folio are already in the uptodate state, so
> > there is no need to go deep into the critical section of state_lock to
> > execute bitmap_set().
> > 
> > Although state_lock may not have significant lock contention due to
> > folio lock, this patch at least reduces the number of instructions.
> > 
> > Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> > ---
> >  fs/iomap/buffered-io.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 3729391a18f3..fb4519158f3a 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -71,6 +71,9 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
> >  	unsigned long flags;
> >  	bool uptodate = true;
> >  
> > +	if (folio_test_uptodate(folio))
> > +		return;
> 
> Looks fine, but how exhaustively have you tested this with heavy IO
> workloads?  I /think/ it's the case that folios always creep towards
> ifs_is_fully_uptodate() == true state and once they've gotten there
> never go back.  But folio state bugs are tricky to detect once they've
> crept in.

I tested fio, ltp and xfstests combined for about 30 hours. The command
used for fio test is:

  fio --name=4k-rw \
    --filename=/data2/testfile \
    --size=1G \
    --bs=4096 \
    --ioengine=libaio \
    --iodepth=32 \
    --rw=randrw \
    --direct=0 \
    --buffered=1 \
    --numjobs=16 \
    --runtime=60 \
    --time_based \
    --group_reporting

ltp and xfstests showed no noticeable errors caused by this patch.

thanks,
Jinliang Zheng. :)

> 
> --D
> 
> > +
> >  	if (ifs) {
> >  		spin_lock_irqsave(&ifs->state_lock, flags);
> >  		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> > -- 
> > 2.49.0
> > 
> >