Re: [PATCH 12/13] iomap: submit ioends immediately

Darrick J. Wong
2023-11-29T05:14:39Z

On Sun, Nov 26, 2023 at 01:47:19PM +0100, Christoph Hellwig wrote:
> Currently the writeback code delays submitting fill ioends until we
> reach the end of the folio. The reason for that is that otherwise
> the end I/O handler could clear the writeback bit before we've even
> finished submitting all I/O for the folio.
>
> Add a bias to ifs->write_bytes_pending while we are submitting I/O
> for a folio so that it never reaches zero until all I/O is completed
> to prevent the early writeback bit clearing, and remove the now
> superfluous submit_list.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks ok, we'll see what happens in the last patch...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/iomap/buffered-io.c | 157 ++++++++++++++++++++---------------------
> 1 file changed, 75 insertions(+), 82 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 9f223820f60d22..a01b0686e7c8a0 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1620,30 +1620,34 @@ static void iomap_writepage_end_bio(struct bio *bio)
> * Submit the final bio for an ioend.
> *
> * If @error is non-zero, it means that we have a situation where some part of
> - * the submission process has failed after we've marked pages for writeback
> - * and unlocked them. In this situation, we need to fail the bio instead of
> - * submitting it. This typically only happens on a filesystem shutdown.
> + * the submission process has failed after we've marked pages for writeback.
> + * We cannot cancel ioend directly in that case, so call the bio end I/O handler
> + * with the error status here to run the normal I/O completion handler to clear
> + * the writeback bit and let the file system proess the errors.
> */
> -static int
> -iomap_submit_ioend(struct iomap_writepage_ctx *wpc, struct iomap_ioend *ioend,
> - int error)
> +static int iomap_submit_ioend(struct iomap_writepage_ctx *wpc, int error)
> {
> + if (!wpc->ioend)
> + return error;
> +
> + /*
> + * Let the file systems prepare the I/O submission and hook in an I/O
> + * comletion handler. This also needs to happen in case after a
> + * failure happened so that the file system end I/O handler gets called
> + * to clean up.
> + */
> if (wpc->ops->prepare_ioend)
> - error = wpc->ops->prepare_ioend(ioend, error);
> + error = wpc->ops->prepare_ioend(wpc->ioend, error);
> +
> if (error) {
> - /*
> - * If we're failing the IO now, just mark the ioend with an
> - * error and finish it. This will run IO completion immediately
> - * as there is only one reference to the ioend at this point in
> - * time.
> - */
> - ioend->io_bio.bi_status = errno_to_blk_status(error);
> - bio_endio(&ioend->io_bio);
> - return error;
> + wpc->ioend->io_bio.bi_status = errno_to_blk_status(error);
> + bio_endio(&wpc->ioend->io_bio);
> + } else {
> + submit_bio(&wpc->ioend->io_bio);
> }
>
> - submit_bio(&ioend->io_bio);
> - return 0;
> + wpc->ioend = NULL;
> + return error;
> }
>
> static struct iomap_ioend *iomap_alloc_ioend(struct iomap_writepage_ctx *wpc,
> @@ -1698,19 +1702,28 @@ iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset)
> /*
> * Test to see if we have an existing ioend structure that we could append to
> * first; otherwise finish off the current ioend and start another.
> + *
> + * If a new ioend is created and cached, the old ioend is submitted to the block
> + * layer instantly. Batching optimisations are provided by higher level block
> + * plugging.
> + *
> + * At the end of a writeback pass, there will be a cached ioend remaining on the
> + * writepage context that the caller will need to submit.
> */
> -static void iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
> +static int iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
> struct writeback_control *wbc, struct folio *folio,
> - struct inode *inode, loff_t pos, struct list_head *iolist)
> + struct inode *inode, loff_t pos)
> {
> struct iomap_folio_state *ifs = folio->private;
> unsigned len = i_blocksize(inode);
> size_t poff = offset_in_folio(folio, pos);
> + int error;
>
> if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, pos)) {
> new_ioend:
> - if (wpc->ioend)
> - list_add(&wpc->ioend->io_list, iolist);
> + error = iomap_submit_ioend(wpc, 0);
> + if (error)
> + return error;
> wpc->ioend = iomap_alloc_ioend(wpc, wbc, inode, pos);
> }
>
> @@ -1721,12 +1734,12 @@ static void iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
> atomic_add(len, &ifs->write_bytes_pending);
> wpc->ioend->io_size += len;
> wbc_account_cgroup_owner(wbc, &folio->page, len);
> + return 0;
> }
>
> static int iomap_writepage_map_blocks(struct iomap_writepage_ctx *wpc,
> struct writeback_control *wbc, struct folio *folio,
> - struct inode *inode, u64 pos, unsigned *count,
> - struct list_head *submit_list)
> + struct inode *inode, u64 pos, unsigned *count)
> {
> int error;
>
> @@ -1743,7 +1756,7 @@ static int iomap_writepage_map_blocks(struct iomap_writepage_ctx *wpc,
> case IOMAP_HOLE:
> break;
> default:
> - iomap_add_to_ioend(wpc, wbc, folio, inode, pos, submit_list);
> + iomap_add_to_ioend(wpc, wbc, folio, inode, pos);
> (*count)++;
> }
>
> @@ -1820,35 +1833,21 @@ static bool iomap_writepage_handle_eof(struct folio *folio, struct inode *inode,
> return true;
> }
>
> -/*
> - * We implement an immediate ioend submission policy here to avoid needing to
> - * chain multiple ioends and hence nest mempool allocations which can violate
> - * the forward progress guarantees we need to provide. The current ioend we're
> - * adding blocks to is cached in the writepage context, and if the new block
> - * doesn't append to the cached ioend, it will create a new ioend and cache that
> - * instead.
> - *
> - * If a new ioend is created and cached, the old ioend is returned and queued
> - * locally for submission once the entire page is processed or an error has been
> - * detected. While ioends are submitted immediately after they are completed,
> - * batching optimisations are provided by higher level block plugging.
> - *
> - * At the end of a writeback pass, there will be a cached ioend remaining on the
> - * writepage context that the caller will need to submit.
> - */
> static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> struct writeback_control *wbc, struct folio *folio)
> {
> struct iomap_folio_state *ifs = folio->private;
> struct inode *inode = folio->mapping->host;
> - struct iomap_ioend *ioend, *next;
> unsigned len = i_blocksize(inode);
> unsigned nblocks = i_blocks_per_folio(inode, folio);
> u64 pos = folio_pos(folio);
> u64 end_pos = pos + folio_size(folio);
> unsigned count = 0;
> int error = 0, i;
> - LIST_HEAD(submit_list);
> +
> + WARN_ON_ONCE(!folio_test_locked(folio));
> + WARN_ON_ONCE(folio_test_dirty(folio));
> + WARN_ON_ONCE(folio_test_writeback(folio));
>
> trace_iomap_writepage(inode, pos, folio_size(folio));
>
> @@ -1858,12 +1857,27 @@ static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> }
> WARN_ON_ONCE(end_pos <= pos);
>
> - if (!ifs && nblocks > 1) {
> - ifs = ifs_alloc(inode, folio, 0);
> - iomap_set_range_dirty(folio, 0, end_pos - pos);
> + if (nblocks > 1) {
> + if (!ifs) {
> + ifs = ifs_alloc(inode, folio, 0);
> + iomap_set_range_dirty(folio, 0, end_pos - pos);
> + }
> +
> + /*
> + * Keep the I/O completion handler from clearing the writeback
> + * bit until we have submitted all blocks by adding a bias to
> + * ifs->write_bytes_pending, which is dropped after submitting
> + * all blocks.
> + */
> + WARN_ON_ONCE(atomic_read(&ifs->write_bytes_pending) != 0);
> + atomic_inc(&ifs->write_bytes_pending);
> }
>
> - WARN_ON_ONCE(ifs && atomic_read(&ifs->write_bytes_pending) != 0);
> + /*
> + * Set the writeback bit ASAP, as the I/O completion for the single
> + * block per folio case happen hit as soon as we're submitting the bio.
> + */
> + folio_start_writeback(folio);
>
> /*
> * Walk through the folio to find areas to write back. If we
> @@ -1874,18 +1888,13 @@ static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> if (ifs && !ifs_block_is_dirty(folio, ifs, i))
> continue;
> error = iomap_writepage_map_blocks(wpc, wbc, folio, inode, pos,
> - &count, &submit_list);
> + &count);
> if (error)
> break;
> }
> if (count)
> wpc->nr_folios++;
>
> - WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
> - WARN_ON_ONCE(!folio_test_locked(folio));
> - WARN_ON_ONCE(folio_test_writeback(folio));
> - WARN_ON_ONCE(folio_test_dirty(folio));
> -
> /*
> * We can have dirty bits set past end of file in page_mkwrite path
> * while mapping the last partial folio. Hence it's better to clear
> @@ -1893,35 +1902,21 @@ static int iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> */
> iomap_clear_range_dirty(folio, 0, folio_size(folio));
>
> - if (error && !count) {
> - folio_unlock(folio);
> - goto done;
> - }
> -
> - folio_start_writeback(folio);
> - folio_unlock(folio);
> -
> /*
> - * Preserve the original error if there was one; catch
> - * submission errors here and propagate into subsequent ioend
> - * submissions.
> + * Usually the writeback bit is cleared by the I/O completion handler.
> + * But we may end up either not actually writing any blocks, or (when
> + * there are multiple blocks in a folio) all I/O might have finished
> + * already at this point. In that case we need to clear the writeback
> + * bit ourselves right after unlocking the page.
> */
> - list_for_each_entry_safe(ioend, next, &submit_list, io_list) {
> - int error2;
> -
> - list_del_init(&ioend->io_list);
> - error2 = iomap_submit_ioend(wpc, ioend, error);
> - if (error2 && !error)
> - error = error2;
> + folio_unlock(folio);
> + if (ifs) {
> + if (atomic_dec_and_test(&ifs->write_bytes_pending))
> + folio_end_writeback(folio);
> + } else {
> + if (!count)
> + folio_end_writeback(folio);
> }
> -
> - /*
> - * We can end up here with no error and nothing to write only if we race
> - * with a partial page truncate on a sub-page block sized filesystem.
> - */
> - if (!count)
> - folio_end_writeback(folio);
> -done:
> mapping_set_error(inode->i_mapping, error);
> return error;
> }
> @@ -1941,9 +1936,7 @@ iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
>
> wpc->ops = ops;
> ret = write_cache_pages(mapping, wbc, iomap_do_writepage, wpc);
> - if (!wpc->ioend)
> - return ret;
> - return iomap_submit_ioend(wpc, wpc->ioend, ret);
> + return iomap_submit_ioend(wpc, ret);
> }
> EXPORT_SYMBOL_GPL(iomap_writepages);
>
> --
> 2.39.2
>
>