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Message-ID: <20231221112453.4gggfki67lxwtor7@quack3>
Date: Thu, 21 Dec 2023 12:24:53 +0100
From: Jan Kara <jack@...e.cz>
To: Christoph Hellwig <hch@....de>
Cc: linux-mm@...ck.org, "Matthew Wilcox (Oracle)" <willy@...radead.org>,
Jan Kara <jack@...e.com>, David Howells <dhowells@...hat.com>,
Brian Foster <bfoster@...hat.com>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/17] writeback: Simplify the loops in
write_cache_pages()
On Mon 18-12-23 16:35:45, Christoph Hellwig wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>
> Collapse the two nested loops into one. This is needed as a step
> towards turning this into an iterator.
>
> Note that this drops the "index <= end" check in the previous outer loop
> and just relies on filemap_get_folios_tag() to return 0 entries when
> index > end. This actually has a subtle implication when end == -1
> because then the returned index will be -1 as well and thus if there is
> page present on index -1, we could be looping indefinitely. But as the
> comment in filemap_get_folios_tag documents this as already broken anyway
> we should not worry about it here either. The fix for that would
> probably a change to the filemap_get_folios_tag() calling convention.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> [hch: updated the commit log based on feedback from Jan Kara]
> Signed-off-by: Christoph Hellwig <hch@....de>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> mm/page-writeback.c | 94 ++++++++++++++++++++++-----------------------
> 1 file changed, 46 insertions(+), 48 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2a004c0b9bdfbf..c7983ea3040be4 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2473,6 +2473,7 @@ int write_cache_pages(struct address_space *mapping,
> {
> int error;
> pgoff_t end; /* Inclusive */
> + int i = 0;
>
> if (wbc->range_cyclic) {
> wbc->index = mapping->writeback_index; /* prev offset */
> @@ -2487,63 +2488,60 @@ int write_cache_pages(struct address_space *mapping,
> folio_batch_init(&wbc->fbatch);
> wbc->err = 0;
>
> - while (wbc->index <= end) {
> - int i;
> -
> - writeback_get_batch(mapping, wbc);
> + for (;;) {
> + struct folio *folio;
> + unsigned long nr;
>
> + if (i == wbc->fbatch.nr) {
> + writeback_get_batch(mapping, wbc);
> + i = 0;
> + }
> if (wbc->fbatch.nr == 0)
> break;
>
> - for (i = 0; i < wbc->fbatch.nr; i++) {
> - struct folio *folio = wbc->fbatch.folios[i];
> - unsigned long nr;
> + folio = wbc->fbatch.folios[i++];
>
> - folio_lock(folio);
> - if (!should_writeback_folio(mapping, wbc, folio)) {
> - folio_unlock(folio);
> - continue;
> - }
> + folio_lock(folio);
> + if (!should_writeback_folio(mapping, wbc, folio)) {
> + folio_unlock(folio);
> + continue;
> + }
>
> - trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
> + trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
>
> - error = writepage(folio, wbc, data);
> - nr = folio_nr_pages(folio);
> - wbc->nr_to_write -= nr;
> + error = writepage(folio, wbc, data);
> + nr = folio_nr_pages(folio);
> + wbc->nr_to_write -= nr;
>
> - /*
> - * 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;
> - }
> -
> - if (error && !wbc->err)
> - wbc->err = error;
> + /*
> + * 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 sync we have to keep going until we
> - * have written all the folios we tagged for writeback
> - * prior to entering this 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 error;
> - }
> + if (error && !wbc->err)
> + wbc->err = error;
> +
> + /*
> + * For integrity sync we have to keep going until we have
> + * written all the folios we tagged for writeback prior to
> + * entering this 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 error;
> }
> }
>
> --
> 2.39.2
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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