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

[Dave Chinner <dgc@kernel.org>]

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