Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Christoph Hellwig]

On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@gmail.com wrote:
> From: Jinliang Zheng <alexjlzheng@tencent.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
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@tencent.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---

Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Matthew Wilcox]

On Thu, Jul 03, 2025 at 06:52:44AM -0700, Christoph Hellwig wrote:
> On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@gmail.com wrote:
> > From: Jinliang Zheng <alexjlzheng@tencent.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
> 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.

Once a folio is uptodate, there is no route back to !uptodate without
going through the removal of the folio from the page cache.  The read()
path relies on this for example; once it has a refcount on the folio,
and has checked the uptodate bit, it will copy the contents to userspace.

Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Jinliang Zheng]

On Thu, 3 Jul 2025 18:34:20 +0100, Matthew Wilcox wrote:
> On Thu, Jul 03, 2025 at 06:52:44AM -0700, Christoph Hellwig wrote:
> > On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@gmail.com wrote:
> > > From: Jinliang Zheng <alexjlzheng@tencent.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
> > 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.
> 
> Once a folio is uptodate, there is no route back to !uptodate without
> going through the removal of the folio from the page cache.  The read()
> path relies on this for example; once it has a refcount on the folio,
> and has checked the uptodate bit, it will copy the contents to userspace.

I agree, and this aligns with my perspective. Thank you for confirming this.

Jinliang Zheng. :)

Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Jinliang Zheng]

On Thu, 3 Jul 2025 06:52:44 -0700, Christoph Hellwig wrote:
> On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@gmail.com wrote:
> > From: Jinliang Zheng <alexjlzheng@tencent.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@tencent.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---

Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Christoph Hellwig]

On Wed, Jul 09, 2025 at 11:30:42AM +0800, Jinliang Zheng wrote:
> 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().

Yes, that seems fine.  Can you update the commit message with some of
the insights from this discussion, and with that the patch should be
fine.