[PATCH] iomap: avoid memset iomap when iter is done

[PATCH] iomap: avoid memset iomap when iter is done

[Fengnan Chang]

When iomap_iter() finishes its iteration (returns <= 0), it is no longer
necessary to memset the entire iomap and srcmap structures.

In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
where the majority of I/Os complete in a single extent map, this wasted
memory write bandwidth, as the caller will just discard the iterator.

Use this command to test:
taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
-n1 -P1 /mnt/testfile
IOPS improve about 5% on ext4 and XFS.

However, we MUST still call iomap_iter_reset_iomap() to release the
folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
references. Therefore, split the cleanup logic: always release the
folio_batch, but skip the memset() when ret <= 0.

Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
---
 fs/iomap/iter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
index c04796f6e57f..91eb5e6165ff 100644
--- a/fs/iomap/iter.c
+++ b/fs/iomap/iter.c
@@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
 	}
 
 	iter->status = 0;
-	memset(&iter->iomap, 0, sizeof(iter->iomap));
-	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
 }
 
 /* Advance the current iterator position and decrement the remaining length */
@@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 	if (ret <= 0)
 		return ret;
 
+	memset(&iter->iomap, 0, sizeof(iter->iomap));
+	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
+
 begin:
 	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
 			       &iter->iomap, &iter->srcmap);
-- 
2.39.5 (Apple Git-154)

Re: [PATCH] iomap: avoid memset iomap when iter is done

[Brian Foster]

On Thu, Apr 16, 2026 at 11:06:42AM +0800, Fengnan Chang wrote:
> When iomap_iter() finishes its iteration (returns <= 0), it is no longer
> necessary to memset the entire iomap and srcmap structures.
> 
> In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
> where the majority of I/Os complete in a single extent map, this wasted
> memory write bandwidth, as the caller will just discard the iterator.
> 
> Use this command to test:
> taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
> -n1 -P1 /mnt/testfile
> IOPS improve about 5% on ext4 and XFS.
> 
> However, we MUST still call iomap_iter_reset_iomap() to release the
> folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
> references. Therefore, split the cleanup logic: always release the
> folio_batch, but skip the memset() when ret <= 0.
> 
> Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> ---
>  fs/iomap/iter.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> index c04796f6e57f..91eb5e6165ff 100644
> --- a/fs/iomap/iter.c
> +++ b/fs/iomap/iter.c
> @@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
>  	}
>  
>  	iter->status = 0;
> -	memset(&iter->iomap, 0, sizeof(iter->iomap));
> -	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
>  }
>  
>  /* Advance the current iterator position and decrement the remaining length */
> @@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
>  	if (ret <= 0)
>  		return ret;
>  
> +	memset(&iter->iomap, 0, sizeof(iter->iomap));
> +	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> +

This seems reasonable to me in principle, but it feels a little odd to
leave a reset helper that doesn't really do a "reset." I wonder if this
should be refactored into an iomap_iter_complete() (i.e. "complete an
iteration") helper that includes the ret assignment logic just above the
reset call and returns it, and then maybe leave a oneline comment above
the memset so somebody doesn't blindly fold it back in the future. So
for example:

	ret = iomap_iter_complete(iter);
	if (ret <= 0)
		return ret;

	/* save cycles and only clear the mappings if we plan to iterate */
	memset(..);
	...

We'd probably have to recheck some of the iter state within the new
helper, but that doesn't seem like a big deal to me. Thoughts?

Brian

>  begin:
>  	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
>  			       &iter->iomap, &iter->srcmap);
> -- 
> 2.39.5 (Apple Git-154)
> 
> 

Re: [PATCH] iomap: avoid memset iomap when iter is done

[Darrick J. Wong]

On Thu, Apr 16, 2026 at 09:27:22AM -0400, Brian Foster wrote:
> On Thu, Apr 16, 2026 at 11:06:42AM +0800, Fengnan Chang wrote:
> > When iomap_iter() finishes its iteration (returns <= 0), it is no longer
> > necessary to memset the entire iomap and srcmap structures.
> > 
> > In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
> > where the majority of I/Os complete in a single extent map, this wasted
> > memory write bandwidth, as the caller will just discard the iterator.
> > 
> > Use this command to test:
> > taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
> > -n1 -P1 /mnt/testfile
> > IOPS improve about 5% on ext4 and XFS.
> > 
> > However, we MUST still call iomap_iter_reset_iomap() to release the
> > folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
> > references. Therefore, split the cleanup logic: always release the
> > folio_batch, but skip the memset() when ret <= 0.
> > 
> > Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> > ---
> >  fs/iomap/iter.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> > index c04796f6e57f..91eb5e6165ff 100644
> > --- a/fs/iomap/iter.c
> > +++ b/fs/iomap/iter.c
> > @@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
> >  	}
> >  
> >  	iter->status = 0;
> > -	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > -	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> >  }
> >  
> >  /* Advance the current iterator position and decrement the remaining length */
> > @@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
> >  	if (ret <= 0)
> >  		return ret;
> >  
> > +	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > +	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > +
> 
> This seems reasonable to me in principle, but it feels a little odd to
> leave a reset helper that doesn't really do a "reset." I wonder if this
> should be refactored into an iomap_iter_complete() (i.e. "complete an
> iteration") helper that includes the ret assignment logic just above the
> reset call and returns it, and then maybe leave a oneline comment above
> the memset so somebody doesn't blindly fold it back in the future. So
> for example:
> 
> 	ret = iomap_iter_complete(iter);
> 	if (ret <= 0)
> 		return ret;
> 
> 	/* save cycles and only clear the mappings if we plan to iterate */
> 	memset(..);
> 	...
> 
> We'd probably have to recheck some of the iter state within the new
> helper, but that doesn't seem like a big deal to me. Thoughts?

What kind of computer is this where there's a 5% hit to iops from a
memset of ~150 bytes?

--D

> Brian
> 
> >  begin:
> >  	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
> >  			       &iter->iomap, &iter->srcmap);
> > -- 
> > 2.39.5 (Apple Git-154)
> > 
> > 
> 

Re: [PATCH] iomap: avoid memset iomap when iter is done

[Dave Chinner]

On Thu, Apr 16, 2026 at 08:27:05AM -0700, Darrick J. Wong wrote:
> On Thu, Apr 16, 2026 at 09:27:22AM -0400, Brian Foster wrote:
> > On Thu, Apr 16, 2026 at 11:06:42AM +0800, Fengnan Chang wrote:
> > > When iomap_iter() finishes its iteration (returns <= 0), it is no longer
> > > necessary to memset the entire iomap and srcmap structures.
> > > 
> > > In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
> > > where the majority of I/Os complete in a single extent map, this wasted
> > > memory write bandwidth, as the caller will just discard the iterator.
> > > 
> > > Use this command to test:
> > > taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
> > > -n1 -P1 /mnt/testfile
> > > IOPS improve about 5% on ext4 and XFS.
> > > 
> > > However, we MUST still call iomap_iter_reset_iomap() to release the
> > > folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
> > > references. Therefore, split the cleanup logic: always release the
> > > folio_batch, but skip the memset() when ret <= 0.
> > > 
> > > Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> > > ---
> > >  fs/iomap/iter.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> > > index c04796f6e57f..91eb5e6165ff 100644
> > > --- a/fs/iomap/iter.c
> > > +++ b/fs/iomap/iter.c
> > > @@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
> > >  	}
> > >  
> > >  	iter->status = 0;
> > > -	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > > -	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > >  }
> > >  
> > >  /* Advance the current iterator position and decrement the remaining length */
> > > @@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
> > >  	if (ret <= 0)
> > >  		return ret;
> > >  
> > > +	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > > +	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > > +
> > 
> > This seems reasonable to me in principle, but it feels a little odd to
> > leave a reset helper that doesn't really do a "reset." I wonder if this
> > should be refactored into an iomap_iter_complete() (i.e. "complete an
> > iteration") helper that includes the ret assignment logic just above the
> > reset call and returns it, and then maybe leave a oneline comment above
> > the memset so somebody doesn't blindly fold it back in the future. So
> > for example:
> > 
> > 	ret = iomap_iter_complete(iter);
> > 	if (ret <= 0)
> > 		return ret;
> > 
> > 	/* save cycles and only clear the mappings if we plan to iterate */
> > 	memset(..);
> > 	...
> > 
> > We'd probably have to recheck some of the iter state within the new
> > helper, but that doesn't seem like a big deal to me. Thoughts?
> 
> What kind of computer is this where there's a 5% hit to iops from a
> memset of ~150 bytes?

Even small costs can have a big impact when you have to pay it many
times.

i.e. 2 million IOPS * 2 * 72 bytes per IO = 288MB/s of memory being
zeroed unnecessarily in very small (inefficient) chunks.

That's definitely enough to cause a 5% drop in IOPS when the
workload is CPU or memory bandwidth bound....

-Dave.
-- 
Dave Chinner
dgc@kernel.org

Re: [PATCH] iomap: avoid memset iomap when iter is done

[changfengnan]

> From: "Dave Chinner"<dgc@kernel.org>
> Date:  Fri, Apr 17, 2026, 06:42
> Subject:  Re: [PATCH] iomap: avoid memset iomap when iter is done
> To: "Darrick J. Wong"<djwong@kernel.org>
> Cc: "Brian Foster"<bfoster@redhat.com>, "Fengnan Chang"<fengnanchang@gmail.com>, <brauner@kernel.org>, <hch@infradead.org>, <linux-xfs@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>, <linux-ext4@vger.kernel.org>, <lidiangang@bytedance.com>, "Fengnan Chang"<changfengnan@bytedance.com>
> On Thu, Apr 16, 2026 at 08:27:05AM -0700, Darrick J. Wong wrote:
> > On Thu, Apr 16, 2026 at 09:27:22AM -0400, Brian Foster wrote:
> > > On Thu, Apr 16, 2026 at 11:06:42AM +0800, Fengnan Chang wrote:
> > > > When iomap_iter() finishes its iteration (returns <= 0), it is no longer
> > > > necessary to memset the entire iomap and srcmap structures.
> > > > 
> > > > In high-IOPS scenarios (like 4k randread NVMe polling with io_uring),
> > > > where the majority of I/Os complete in a single extent map, this wasted
> > > > memory write bandwidth, as the caller will just discard the iterator.
> > > > 
> > > > Use this command to test:
> > > > taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
> > > > -n1 -P1 /mnt/testfile
> > > > IOPS improve about 5% on ext4 and XFS.
> > > > 
> > > > However, we MUST still call iomap_iter_reset_iomap() to release the
> > > > folio_batch if IOMAP_F_FOLIO_BATCH is set, otherwise we leak page
> > > > references. Therefore, split the cleanup logic: always release the
> > > > folio_batch, but skip the memset() when ret <= 0.
> > > > 
> > > > Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> > > > ---
> > > >  fs/iomap/iter.c | 5 +++--
> > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> > > > index c04796f6e57f..91eb5e6165ff 100644
> > > > --- a/fs/iomap/iter.c
> > > > +++ b/fs/iomap/iter.c
> > > > @@ -15,8 +15,6 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
> > > >          }
> > > >  
> > > >          iter->status = 0;
> > > > -        memset(&iter->iomap, 0, sizeof(iter->iomap));
> > > > -        memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > > >  }
> > > >  
> > > >  /* Advance the current iterator position and decrement the remaining length */
> > > > @@ -106,6 +104,9 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
> > > >          if (ret <= 0)
> > > >                  return ret;
> > > >  
> > > > +        memset(&iter->iomap, 0, sizeof(iter->iomap));
> > > > +        memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > > > +
> > > 
> > > This seems reasonable to me in principle, but it feels a little odd to
> > > leave a reset helper that doesn't really do a "reset." I wonder if this
> > > should be refactored into an iomap_iter_complete() (i.e. "complete an
> > > iteration") helper that includes the ret assignment logic just above the
> > > reset call and returns it, and then maybe leave a oneline comment above
> > > the memset so somebody doesn't blindly fold it back in the future. So
> > > for example:
> > > 
> > >         ret = iomap_iter_complete(iter);
> > >         if (ret <= 0)
> > >                 return ret;
> > > 
> > >         /* save cycles and only clear the mappings if we plan to iterate */
> > >         memset(..);
> > >         ...
> > > 
> > > We'd probably have to recheck some of the iter state within the new
> > > helper, but that doesn't seem like a big deal to me. Thoughts?
> > 
> > What kind of computer is this where there's a 5% hit to iops from a
> > memset of ~150 bytes?
> 
> Even small costs can have a big impact when you have to pay it many
> times.
> 
> i.e. 2 million IOPS * 2 * 72 bytes per IO = 288MB/s of memory being
> zeroed unnecessarily in very small (inefficient) chunks.
> 
> That's definitely enough to cause a 5% drop in IOPS when the
> workload is CPU or memory bandwidth bound....

My CPU is  Intel(R) Xeon(R) Platinum 8457C with 1TB RAM, NVMe is 
PS1010 and the system is very idle, only run one command with one core:
taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 \
-X1 -n1 -P1 /mnt/mytest，IOPS  improved from 1.9M to 2.0M.
My test is a CPU bound case，these two memset operations should
have taken only about 15 nanoseconds. at 2.0 million IOPS, the total 
CPU time budget per I/O operation averages just 500 nanoseconds.
Taking into account the impact on the pipeline and the L1 cache, a 5%
improvement is reasonable.

> 
> -Dave.
> -- 
> Dave Chinner
> dgc@kernel.org
> 

Re: [PATCH] iomap: avoid memset iomap when iter is done

[Mateusz Guzik]

On Thu, Apr 16, 2026 at 08:27:05AM -0700, Darrick J. Wong wrote:
> What kind of computer is this where there's a 5% hit to iops from a
> memset of ~150 bytes?
> 

Such a memset is not free in its own right, but here the real problem is
*how* it is done: by default gcc emits inlined 'rep stosq' which has
utterly horrid performance (same as 'rep movsq') and in this particular
case the 2 memsets in the source code result in 2 separate 'rep movsq'
invocations.

I complained about it numerous times, see this to get a taste:
https://lore.kernel.org/all/20250605164733.737543-1-mjguzik@gmail.com/

Also see
https://lore.kernel.org/all/202504181042.54ea2b8a-lkp@intel.com/

If memory serves right gcc is going to fix it, so this is largely going
to clear itself down the road.

Re: [PATCH] iomap: avoid memset iomap when iter is done

[changfengnan]

> From: "Mateusz Guzik"<mjguzik@gmail.com>
> Date:  Fri, Apr 17, 2026, 07:21
> Subject:  Re: [PATCH] iomap: avoid memset iomap when iter is done
> To: "Darrick J. Wong"<djwong@kernel.org>
> Cc: "Brian Foster"<bfoster@redhat.com>, "Fengnan Chang"<fengnanchang@gmail.com>, <brauner@kernel.org>, <hch@infradead.org>, <linux-xfs@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>, <linux-ext4@vger.kernel.org>, <lidiangang@bytedance.com>, "Fengnan Chang"<changfengnan@bytedance.com>
> On Thu, Apr 16, 2026 at 08:27:05AM -0700, Darrick J. Wong wrote:
> > What kind of computer is this where there's a 5% hit to iops from a
> > memset of ~150 bytes?
> > 
> 
> Such a memset is not free in its own right, but here the real problem is
> *how* it is done: by default gcc emits inlined 'rep stosq' which has
> utterly horrid performance (same as 'rep movsq') and in this particular
> case the 2 memsets in the source code result in 2 separate 'rep movsq'
> invocations.
Yes,  two 'rep stosq'.
> 
> I complained about it numerous times, see this to get a taste:
> https://lore.kernel.org/all/20250605164733.737543-1-mjguzik@gmail.com/
> 
> Also see
> https://lore.kernel.org/all/202504181042.54ea2b8a-lkp@intel.com/
> 
> If memory serves right gcc is going to fix it, so this is largely going
> to clear itself down the road.
Sounds good, I believe there are other similar issues that haven't been
discovered yet. In this case, removing `memset` is the best option, since
we don't need it.

> 

Re: [PATCH] iomap: avoid memset iomap when iter is done

[Christoph Hellwig]

On Thu, Apr 16, 2026 at 09:27:22AM -0400, Brian Foster wrote:
> This seems reasonable to me in principle, but it feels a little odd to
> leave a reset helper that doesn't really do a "reset."

Agreed.

> I wonder if this
> should be refactored into an iomap_iter_complete() (i.e. "complete an
> iteration") helper that includes the ret assignment logic just above the
> reset call and returns it, and then maybe leave a oneline comment above
> the memset so somebody doesn't blindly fold it back in the future. So
> for example:

What about just killing iomap_iter_reset_iomap in it's current form
instead?  Move the batch reset logic into a little helper if we care,
and just inline the rest into the only claler to side-step the issue.

Re: [PATCH] iomap: avoid memset iomap when iter is done

[changfengnan]

> From: "Christoph Hellwig"<hch@infradead.org>
> Date:  Fri, Apr 17, 2026, 15:27
> Subject:  Re: [PATCH] iomap: avoid memset iomap when iter is done
> To: "Brian Foster"<bfoster@redhat.com>
> Cc: "Fengnan Chang"<fengnanchang@gmail.com>, <brauner@kernel.org>, <djwong@kernel.org>, <hch@infradead.org>, <linux-xfs@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>, <linux-ext4@vger.kernel.org>, <lidiangang@bytedance.com>, "Fengnan Chang"<changfengnan@bytedance.com>
> On Thu, Apr 16, 2026 at 09:27:22AM -0400, Brian Foster wrote:
> > This seems reasonable to me in principle, but it feels a little odd to
> > leave a reset helper that doesn't really do a "reset."
> 
> Agreed.
> 
> > I wonder if this
> > should be refactored into an iomap_iter_complete() (i.e. "complete an
> > iteration") helper that includes the ret assignment logic just above the
> > reset call and returns it, and then maybe leave a oneline comment above
> > the memset so somebody doesn't blindly fold it back in the future. So
> > for example:
> 
> What about just killing iomap_iter_reset_iomap in it's current form
> instead?  Move the batch reset logic into a little helper if we care,
> and just inline the rest into the only claler to side-step the issue.
Sounds good, move iter->status out, and then rename 
iomap_iter_reset_iomap to iomap_iter_clean_folio_batch.
>