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

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Message-ID: <20250709033042.249954-1-alexjlzheng@tencent.com>
Date: Wed,  9 Jul 2025 11:30:42 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: hch@...radead.org
Cc: alexjlzheng@...il.com,
	alexjlzheng@...cent.com,
	brauner@...nel.org,
	djwong@...nel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-xfs@...r.kernel.org
Subject: Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

On Thu, 3 Jul 2025 06:52:44 -0700, Christoph Hellwig wrote:
> On Tue, Jul 01, 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.
> 
> That means the uptodate bitmap is stale in that case.  That would

Hi, after days of silence, I re-read this email thread to make sure I
didn't miss something important.

I realized that maybe we are not aligned and I didn't understand your
sentence above. Would you mind explaining your meaning in more detail?

In addition, what I want to say is that once folio_test_uptodate() is
true, all bits in ifs->state are in the uptodate state. So there is no
need to acquire the lock and set it again. This repeated setting happens
in __iomap_write_end().

thanks,
Jinliang Zheng. :)

> only matter if we could clear the folio uptodate bit and still
> expect the page content to survive.  Which sounds dubious and I could
> not find anything relevant grepping the tree, but I'm adding the
> linux-mm list just in case.
> 
> > 
> > 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;
> > +
> >  	if (ifs) {
> >  		spin_lock_irqsave(&ifs->state_lock, flags);
> >  		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> > -- 
> > 2.49.0
> > 
> > 
> ---end quoted text---