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Wong" Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, Christoph Hellwig Subject: Re: [PATCH v5 07/10] iomap: support incremental iomap_iter advances Message-ID: References: <20250205135821.178256-1-bfoster@redhat.com> <20250205135821.178256-8-bfoster@redhat.com> <20250205191016.GQ21808@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250205191016.GQ21808@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On Wed, Feb 05, 2025 at 11:10:16AM -0800, Darrick J. Wong wrote: > On Wed, Feb 05, 2025 at 08:58:18AM -0500, Brian Foster wrote: > > The current iomap_iter iteration model reads the mapping from the > > filesystem, processes the subrange of the operation associated with > > the current mapping, and returns the number of bytes processed back > > to the iteration code. The latter advances the position and > > remaining length of the iter in preparation for the next iteration. > > > > At the _iter() handler level, this tends to produce a processing > > loop where the local code pulls the current position and remaining > > length out of the iter, iterates it locally based on file offset, > > and then breaks out when the associated range has been fully > > processed. > > > > This works well enough for current handlers, but upcoming > > enhancements require a bit more flexibility in certain situations. > > Enhancements for zero range will lead to a situation where the > > processing loop is no longer a pure ascending offset walk, but > > rather dictated by pagecache state and folio lookup. Since folio > > lookup and write preparation occur at different levels, it is more > > difficult to manage position and length outside of the iter. > > > > To provide more flexibility to certain iomap operations, introduce > > support for incremental iomap_iter advances from within the > > operation itself. This allows more granular advances for operations > > that might not use the typical file offset based walk. > > > > Note that the semantics for operations that use incremental advances > > is slightly different than traditional operations. Operations that > > advance the iter directly are expected to return success or failure > > (i.e. 0 or negative error code) in iter.processed rather than the > > number of bytes processed. > > I think this needs to be documented in the code comments for @processed > in iomap.h: > > * @processed: The iteration loop body should set this to a negative > * errno if an error occurs during processing; zero if it advanced > * the iter itself with iomap_iter_advance; or the number of bytes > * processed if it needs iomap_iter to advance the iter. > Note that this would be shortlived content re: my previous comments on the advance going away, but sure I can change it. Brian > --D > > > Signed-off-by: Brian Foster > > Reviewed-by: Christoph Hellwig > > --- > > fs/iomap/iter.c | 32 +++++++++++++++++++++++++------- > > include/linux/iomap.h | 3 +++ > > 2 files changed, 28 insertions(+), 7 deletions(-) > > > > diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c > > index cdba24dbbfd7..9273ef36d5ae 100644 > > --- a/fs/iomap/iter.c > > +++ b/fs/iomap/iter.c > > @@ -35,6 +35,8 @@ static inline void iomap_iter_done(struct iomap_iter *iter) > > WARN_ON_ONCE(iter->iomap.offset + iter->iomap.length <= iter->pos); > > WARN_ON_ONCE(iter->iomap.flags & IOMAP_F_STALE); > > > > + iter->iter_start_pos = iter->pos; > > + > > trace_iomap_iter_dstmap(iter->inode, &iter->iomap); > > if (iter->srcmap.type != IOMAP_HOLE) > > trace_iomap_iter_srcmap(iter->inode, &iter->srcmap); > > @@ -58,6 +60,8 @@ static inline void iomap_iter_done(struct iomap_iter *iter) > > int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops) > > { > > bool stale = iter->iomap.flags & IOMAP_F_STALE; > > + ssize_t advanced = iter->processed > 0 ? iter->processed : 0; > > + u64 olen = iter->len; > > s64 processed; > > int ret; > > > > @@ -66,11 +70,22 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops) > > if (!iter->iomap.length) > > goto begin; > > > > + /* > > + * If iter.processed is zero, the op may still have advanced the iter > > + * itself. Calculate the advanced and original length bytes based on how > > + * far pos has advanced for ->iomap_end(). > > + */ > > + if (!advanced) { > > + advanced = iter->pos - iter->iter_start_pos; > > + olen += advanced; > > + } > > + > > if (ops->iomap_end) { > > - ret = ops->iomap_end(iter->inode, iter->pos, iomap_length(iter), > > - iter->processed > 0 ? iter->processed : 0, > > - iter->flags, &iter->iomap); > > - if (ret < 0 && !iter->processed) > > + ret = ops->iomap_end(iter->inode, iter->iter_start_pos, > > + iomap_length_trim(iter, iter->iter_start_pos, > > + olen), > > + advanced, iter->flags, &iter->iomap); > > + if (ret < 0 && !advanced) > > return ret; > > } > > > > @@ -81,8 +96,11 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops) > > } > > > > /* > > - * Advance the iter and clear state from the previous iteration. Use > > - * iter->len to determine whether to continue onto the next mapping. > > + * Advance the iter and clear state from the previous iteration. This > > + * passes iter->processed because that reflects the bytes processed but > > + * not yet advanced by the iter handler. > > + * > > + * Use iter->len to determine whether to continue onto the next mapping. > > * Explicitly terminate in the case where the current iter has not > > * advanced at all (i.e. no work was done for some reason) unless the > > * mapping has been marked stale and needs to be reprocessed. > > @@ -90,7 +108,7 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops) > > ret = iomap_iter_advance(iter, &processed); > > if (!ret && iter->len > 0) > > ret = 1; > > - if (ret > 0 && !iter->processed && !stale) > > + if (ret > 0 && !advanced && !stale) > > ret = 0; > > iomap_iter_reset_iomap(iter); > > if (ret <= 0) > > diff --git a/include/linux/iomap.h b/include/linux/iomap.h > > index f304c602e5fe..0135a7f8dd83 100644 > > --- a/include/linux/iomap.h > > +++ b/include/linux/iomap.h > > @@ -211,6 +211,8 @@ struct iomap_ops { > > * calls to iomap_iter(). Treat as read-only in the body. > > * @len: The remaining length of the file segment we're operating on. > > * It is updated at the same time as @pos. > > + * @iter_start_pos: The original start pos for the current iomap. Used for > > + * incremental iter advance. > > * @processed: The number of bytes processed by the body in the most recent > > * iteration, or a negative errno. 0 causes the iteration to stop. > > * @flags: Zero or more of the iomap_begin flags above. > > @@ -221,6 +223,7 @@ struct iomap_iter { > > struct inode *inode; > > loff_t pos; > > u64 len; > > + loff_t iter_start_pos; > > s64 processed; > > unsigned flags; > > struct iomap iomap; > > -- > > 2.48.1 > > > > >