Re: [PATCH v4 02/10] iomap: split out iomap check and reset logic from iter advance

[Brian Foster <bfoster@redhat.com>]

On Tue, Feb 04, 2025 at 11:30:56AM -0800, Darrick J. Wong wrote:
> On Tue, Feb 04, 2025 at 08:30:36AM -0500, Brian Foster wrote:
> > In preparation for more granular iomap_iter advancing, break out
> > some of the logic associated with higher level iteration from
> > iomap_advance_iter(). Specifically, factor the iomap reset code into
> > a separate helper and lift the iomap.length check into the calling
> > code, similar to how ->iomap_end() calls are handled.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/iomap/iter.c | 49 ++++++++++++++++++++++++++-----------------------
> >  1 file changed, 26 insertions(+), 23 deletions(-)
> > 
> > diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> > index 3790918646af..731ea7267f27 100644
> > --- a/fs/iomap/iter.c
> > +++ b/fs/iomap/iter.c
> > @@ -7,6 +7,13 @@
> >  #include <linux/iomap.h>
> >  #include "trace.h"
> >  
> > +static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
> > +{
> > +	iter->processed = 0;
> > +	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > +	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> > +}
> > +
> >  /*
> >   * Advance to the next range we need to map.
> >   *
> > @@ -14,32 +21,24 @@
> >   * processed - it was aborted because the extent the iomap spanned may have been
> >   * changed during the operation. In this case, the iteration behaviour is to
> >   * remap the unprocessed range of the iter, and that means we may need to remap
> > - * even when we've made no progress (i.e. iter->processed = 0). Hence the
> > - * "finished iterating" case needs to distinguish between
> > - * (processed = 0) meaning we are done and (processed = 0 && stale) meaning we
> > - * need to remap the entire remaining range.
> > + * even when we've made no progress (i.e. count = 0). Hence the "finished
> > + * iterating" case needs to distinguish between (count = 0) meaning we are done
> > + * and (count = 0 && stale) meaning we need to remap the entire remaining range.
> >   */
> > -static inline int iomap_iter_advance(struct iomap_iter *iter)
> > +static inline int iomap_iter_advance(struct iomap_iter *iter, s64 count)
> >  {
> >  	bool stale = iter->iomap.flags & IOMAP_F_STALE;
> >  	int ret = 1;
> >  
> > -	/* handle the previous iteration (if any) */
> > -	if (iter->iomap.length) {
> > -		if (iter->processed < 0)
> > -			return iter->processed;
> > -		if (WARN_ON_ONCE(iter->processed > iomap_length(iter)))
> > -			return -EIO;
> > -		iter->pos += iter->processed;
> > -		iter->len -= iter->processed;
> > -		if (!iter->len || (!iter->processed && !stale))
> > -			ret = 0;
> > -	}
> > +	if (count < 0)
> > +		return count;
> > +	if (WARN_ON_ONCE(count > iomap_length(iter)))
> > +		return -EIO;
> > +	iter->pos += count;
> > +	iter->len -= count;
> > +	if (!iter->len || (!count && !stale))
> > +		ret = 0;
> >  
> > -	/* clear the per iteration state */
> > -	iter->processed = 0;
> > -	memset(&iter->iomap, 0, sizeof(iter->iomap));
> > -	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> 
> Are there any consequences to not resetting the iter if
> iter->iomap.length is zero?  I think the answer is "no" because callers
> are supposed to initialize the iter with zeroes and filesystems are
> never supposed to return zero-length iomaps from ->begin_iomap, right?
> 

That matches my understanding..

> If the answers are "no" and "yes" then
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> 

Thanks.

Brian

> --D
> 
> >  	return ret;
> >  }
> >  
> > @@ -82,10 +81,14 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
> >  			return ret;
> >  	}
> >  
> > +	/* advance and clear state from the previous iteration */
> >  	trace_iomap_iter(iter, ops, _RET_IP_);
> > -	ret = iomap_iter_advance(iter);
> > -	if (ret <= 0)
> > -		return ret;
> > +	if (iter->iomap.length) {
> > +		ret = iomap_iter_advance(iter, iter->processed);
> > +		iomap_iter_reset_iomap(iter);
> > +		if (ret <= 0)
> > +			return ret;
> > +	}
> >  
> >  	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
> >  			       &iter->iomap, &iter->srcmap);
> > -- 
> > 2.48.1
> > 
> > 
>