Re: [PATCH 19/19] writeback: simplify writeback iteration

Jan Kara
2024-01-31T10:40:47Z

On Wed 31-01-24 08:14:37, Christoph Hellwig wrote:
> On Tue, Jan 30, 2024 at 10:50:16PM +0100, Jan Kara wrote:
> > Well, batch release needs to be only here because if writeback_get_folio()
> > returns NULL, the batch has been already released by it.
>
> Indeed.
>
> > So what would be
> > duplicated is only the error assignment. But I'm fine with the version in
> > the following email and actually somewhat prefer it compared the yet
> > another variant you've sent.
>
> So how about another variant, this is closer to your original suggestion.
> But I've switched around the ordered of the folio or not branches
> from my original patch, and completely reworked and (IMHO) improved the
> comments. it replaces patch 19 instead of being incremental to be
> readable:

Yes, this looks very good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 58b3661f5eac9e..1593a783176ca2 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1985,12 +1985,13 @@ iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
> struct iomap_writepage_ctx *wpc,
> const struct iomap_writeback_ops *ops)
> {
> - struct folio *folio;
> - int ret;
> + struct folio *folio = NULL;
> + int ret = 0;
>
> wpc->ops = ops;
> - for_each_writeback_folio(mapping, wbc, folio, ret)
> + while ((folio = writeback_iter(mapping, wbc, folio, &ret)))
> ret = iomap_do_writepage(folio, wbc, wpc);
> +
> if (!wpc->ioend)
> return ret;
> return iomap_submit_ioend(wpc, wpc->ioend, ret);
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 2416da933440e2..fc4605627496fc 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -367,15 +367,8 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
>
> bool wb_over_bg_thresh(struct bdi_writeback *wb);
>
> -struct folio *writeback_iter_init(struct address_space *mapping,
> - struct writeback_control *wbc);
> -struct folio *writeback_iter_next(struct address_space *mapping,
> - struct writeback_control *wbc, struct folio *folio, int error);
> -
> -#define for_each_writeback_folio(mapping, wbc, folio, error) \
> - for (folio = writeback_iter_init(mapping, wbc); \
> - folio || ((error = wbc->err), false); \
> - folio = writeback_iter_next(mapping, wbc, folio, error))
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error);
>
> typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
> void *data);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 0763c4353a676a..eefcb00cb7b227 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2360,29 +2360,6 @@ void tag_pages_for_writeback(struct address_space *mapping,
> }
> EXPORT_SYMBOL(tag_pages_for_writeback);
>
> -static void writeback_finish(struct address_space *mapping,
> - struct writeback_control *wbc, pgoff_t done_index)
> -{
> - folio_batch_release(&wbc->fbatch);
> -
> - /*
> - * For range cyclic writeback we need to remember where we stopped so
> - * that we can continue there next time we are called. If we hit the
> - * last page and there is more work to be done, wrap back to the start
> - * of the file.
> - *
> - * For non-cyclic writeback we always start looking up at the beginning
> - * of the file if we are called again, which can only happen due to
> - * -ENOMEM from the file system.
> - */
> - if (wbc->range_cyclic) {
> - if (wbc->err || wbc->nr_to_write <= 0)
> - mapping->writeback_index = done_index;
> - else
> - mapping->writeback_index = 0;
> - }
> -}
> -
> static xa_mark_t wbc_to_tag(struct writeback_control *wbc)
> {
> if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> @@ -2442,10 +2419,8 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> filemap_get_folios_tag(mapping, &wbc->index, wbc_end(wbc),
> wbc_to_tag(wbc), &wbc->fbatch);
> folio = folio_batch_next(&wbc->fbatch);
> - if (!folio) {
> - writeback_finish(mapping, wbc, 0);
> + if (!folio)
> return NULL;
> - }
> }
>
> folio_lock(folio);
> @@ -2458,60 +2433,107 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> return folio;
> }
>
> -struct folio *writeback_iter_init(struct address_space *mapping,
> - struct writeback_control *wbc)
> +/**
> + * writepage_iter - iterate folio of a mapping for writeback
> + * @mapping: address space structure to write
> + * @wbc: writeback context
> + * @folio: previously iterated folio (%NULL to start)
> + * @error: in-out pointer for writeback errors (see below)
> + *
> + * This function should be called in a while loop in the ->writepages
> + * implementation and returns the next folio for the writeback operation
> + * described by @wbc on @mapping.
> + *
> + * To start writeback @folio should be passed as NULL, for every following
> + * iteration the folio returned by this function previously should be passed.
> + * @error should contain the error from the previous writeback iteration when
> + * calling writeback_iter.
> + *
> + * Once the writeback described in @wbc has finished, this function will return
> + * %NULL and if there was an error in any iteration restore it to @error.
> + *
> + * Note: callers should not manually break out of the loop using break or goto.
> + */
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error)
> {
> - if (wbc->range_cyclic)
> - wbc->index = mapping->writeback_index; /* prev offset */
> - else
> - wbc->index = wbc->range_start >> PAGE_SHIFT;
> -
> - if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> - tag_pages_for_writeback(mapping, wbc->index, wbc_end(wbc));
> -
> - wbc->err = 0;
> - folio_batch_init(&wbc->fbatch);
> - return writeback_get_folio(mapping, wbc);
> -}
> + if (!folio) {
> + folio_batch_init(&wbc->fbatch);
> + wbc->err = 0;
>
> -struct folio *writeback_iter_next(struct address_space *mapping,
> - struct writeback_control *wbc, struct folio *folio, int error)
> -{
> - unsigned long nr = folio_nr_pages(folio);
> + /*
> + * For range cyclic writeback we remember where we stopped so
> + * that we can continue where we stopped.
> + *
> + * For non-cyclic writeback we always start at the beginning of
> + * the passed in range.
> + */
> + if (wbc->range_cyclic)
> + wbc->index = mapping->writeback_index;
> + else
> + wbc->index = wbc->range_start >> PAGE_SHIFT;
>
> - wbc->nr_to_write -= nr;
> + /*
> + * To avoid livelocks when other processes dirty new pages, we
> + * first tag pages which should be written back and only then
> + * start writing them.
> + *
> + * For data-integrity sync we have to be careful so that we do
> + * not miss some pages (e.g., because some other process has
> + * cleared the TOWRITE tag we set). The rule we follow is that
> + * TOWRITE tag can be cleared only by the process clearing the
> + * DIRTY tag (and submitting the page for I/O).
> + */
> + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> + tag_pages_for_writeback(mapping, wbc->index,
> + wbc_end(wbc));
> + } else {
> + wbc->nr_to_write -= folio_nr_pages(folio);
>
> - /*
> - * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return value.
> - * Eventually all instances should just unlock the folio themselves and
> - * return 0;
> - */
> - if (error == AOP_WRITEPAGE_ACTIVATE) {
> - folio_unlock(folio);
> - error = 0;
> + /*
> + * For integrity writeback we have to keep going until we have
> + * written all the folios we tagged for writeback prior to
> + * entering the writeback loop, even if we run past
> + * wbc->nr_to_write or encounter errors, and just stash away
> + * the first error we encounter in wbc->err so that it can
> + * be retrieved on return.
> + *
> + * This is because the file system may still have state to clear
> + * for each folio. We'll eventually return the first error
> + * encountered.
> + */
> + if (wbc->sync_mode == WB_SYNC_ALL) {
> + if (*error && !wbc->err)
> + wbc->err = *error;
> + } else {
> + if (*error || wbc->nr_to_write <= 0)
> + goto done;
> + }
> }
>
> - if (error && !wbc->err)
> - wbc->err = error;
> + folio = writeback_get_folio(mapping, wbc);
> + if (!folio) {
> + /*
> + * For range cyclic writeback not finding another folios means
> + * that we are at the end of the file. In that case go back
> + * to the start of the file for the next call.
> + */
> + if (wbc->range_cyclic)
> + mapping->writeback_index = 0;
>
> - /*
> - * For integrity sync we have to keep going until we have written all
> - * the folios we tagged for writeback prior to entering the writeback
> - * loop, even if we run past wbc->nr_to_write or encounter errors.
> - * This is because the file system may still have state to clear for
> - * each folio. We'll eventually return the first error encountered.
> - *
> - * For background writeback just push done_index past this folio so that
> - * we can just restart where we left off and media errors won't choke
> - * writeout for the entire file.
> - */
> - if (wbc->sync_mode == WB_SYNC_NONE &&
> - (wbc->err || wbc->nr_to_write <= 0)) {
> - writeback_finish(mapping, wbc, folio->index + nr);
> - return NULL;
> + /*
> + * Return the first error we encountered (if there was any) to
> + * the caller now that we are done.
> + */
> + *error = wbc->err;
> }
> + return folio;
>
> - return writeback_get_folio(mapping, wbc);
> +done:
> + if (wbc->range_cyclic)
> + mapping->writeback_index = folio->index + folio_nr_pages(folio);
> + folio_batch_release(&wbc->fbatch);
> + return NULL;
> }
>
> /**
> @@ -2549,13 +2571,18 @@ int write_cache_pages(struct address_space *mapping,
> struct writeback_control *wbc, writepage_t writepage,
> void *data)
> {
> - struct folio *folio;
> - int error;
> + struct folio *folio = NULL;
> + int error = 0;
>
> - for_each_writeback_folio(mapping, wbc, folio, error)
> + while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> error = writepage(folio, wbc, data);
> + if (error == AOP_WRITEPAGE_ACTIVATE) {
> + folio_unlock(folio);
> + error = 0;
> + }
> + }
>
> - return wbc->err;
> + return error;
> }
> EXPORT_SYMBOL(write_cache_pages);
>
> @@ -2563,13 +2590,17 @@ static int writeback_use_writepage(struct address_space *mapping,
> struct writeback_control *wbc)
> {
> struct blk_plug plug;
> - struct folio *folio;
> - int err;
> + struct folio *folio = NULL;
> + int err = 0;
>
> blk_start_plug(&plug);
> - for_each_writeback_folio(mapping, wbc, folio, err) {
> + while ((folio = writeback_iter(mapping, wbc, folio, &err))) {
> err = mapping->a_ops->writepage(&folio->page, wbc);
> mapping_set_error(mapping, err);
> + if (err == AOP_WRITEPAGE_ACTIVATE) {
> + folio_unlock(folio);
> + err = 0;
> + }
> }
> blk_finish_plug(&plug);
>
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR