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Date:   Thu, 1 Dec 2022 14:43:16 +0530
From:   "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/9] ext4: Move keep_towrite handling to
 ext4_bio_write_page()

On 22/11/30 05:35PM, Jan Kara wrote:
> When we are writing back page but we cannot for some reason write all
> its buffers (e.g. because we cannot allocate blocks in current context) we
> have to keep TOWRITE tag set in the mapping as otherwise racing
> WB_SYNC_ALL writeback that could write these buffers can skip the page
> and result in data loss. We will need this logic for writeback during
> transaction commit so move the logic from ext4_writepage() to
> ext4_bio_write_page().


Nice explaination.
Moving set_page_writeback() and set_page_writeback_keepwrite() to after
identifying any buffers needs to be written back or not is also sweet.
This avoid unnecessary calling of end_page_writeback().

The patch looks good to me. Please feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>



>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/ext4/ext4.h    |  3 +--
>  fs/ext4/inode.c   |  6 ++----
>  fs/ext4/page-io.c | 36 +++++++++++++++++++++---------------
>  3 files changed, 24 insertions(+), 21 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 8d5453852f98..1b3bffc04fd0 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3756,8 +3756,7 @@ extern void ext4_end_io_rsv_work(struct work_struct *work);
>  extern void ext4_io_submit(struct ext4_io_submit *io);
>  extern int ext4_bio_write_page(struct ext4_io_submit *io,
>  			       struct page *page,
> -			       int len,
> -			       bool keep_towrite);
> +			       int len);
>  extern struct ext4_io_end_vec *ext4_alloc_io_end_vec(ext4_io_end_t *io_end);
>  extern struct ext4_io_end_vec *ext4_last_io_end_vec(ext4_io_end_t *io_end);
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 2b5ef1b64249..43eb175d0c1c 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2009,7 +2009,6 @@ static int ext4_writepage(struct page *page,
>  	struct buffer_head *page_bufs = NULL;
>  	struct inode *inode = page->mapping->host;
>  	struct ext4_io_submit io_submit;
> -	bool keep_towrite = false;
>
>  	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb)))) {
>  		folio_invalidate(folio, 0, folio_size(folio));
> @@ -2067,7 +2066,6 @@ static int ext4_writepage(struct page *page,
>  			unlock_page(page);
>  			return 0;
>  		}
> -		keep_towrite = true;
>  	}
>
>  	if (PageChecked(page) && ext4_should_journal_data(inode))
> @@ -2084,7 +2082,7 @@ static int ext4_writepage(struct page *page,
>  		unlock_page(page);
>  		return -ENOMEM;
>  	}
> -	ret = ext4_bio_write_page(&io_submit, page, len, keep_towrite);
> +	ret = ext4_bio_write_page(&io_submit, page, len);
>  	ext4_io_submit(&io_submit);
>  	/* Drop io_end reference we got from init */
>  	ext4_put_io_end_defer(io_submit.io_end);
> @@ -2118,7 +2116,7 @@ static int mpage_submit_page(struct mpage_da_data *mpd, struct page *page)
>  		len = size & ~PAGE_MASK;
>  	else
>  		len = PAGE_SIZE;
> -	err = ext4_bio_write_page(&mpd->io_submit, page, len, false);
> +	err = ext4_bio_write_page(&mpd->io_submit, page, len);
>  	if (!err)
>  		mpd->wbc->nr_to_write--;
>  	mpd->first_page++;
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 4e68ace86f11..4f9ecacd10aa 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -430,8 +430,7 @@ static void io_submit_add_bh(struct ext4_io_submit *io,
>
>  int ext4_bio_write_page(struct ext4_io_submit *io,
>  			struct page *page,
> -			int len,
> -			bool keep_towrite)
> +			int len)
>  {
>  	struct page *bounce_page = NULL;
>  	struct inode *inode = page->mapping->host;
> @@ -441,14 +440,11 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  	int nr_submitted = 0;
>  	int nr_to_submit = 0;
>  	struct writeback_control *wbc = io->io_wbc;
> +	bool keep_towrite = false;
>
>  	BUG_ON(!PageLocked(page));
>  	BUG_ON(PageWriteback(page));
>
> -	if (keep_towrite)
> -		set_page_writeback_keepwrite(page);
> -	else
> -		set_page_writeback(page);
>  	ClearPageError(page);
>
>  	/*
> @@ -483,12 +479,17 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  			if (!buffer_mapped(bh))
>  				clear_buffer_dirty(bh);
>  			/*
> -			 * Keeping dirty some buffer we cannot write? Make
> -			 * sure to redirty the page. This happens e.g. when
> -			 * doing writeout for transaction commit.
> +			 * Keeping dirty some buffer we cannot write? Make sure
> +			 * to redirty the page and keep TOWRITE tag so that
> +			 * racing WB_SYNC_ALL writeback does not skip the page.
> +			 * This happens e.g. when doing writeout for
> +			 * transaction commit.
>  			 */
> -			if (buffer_dirty(bh) && !PageDirty(page))
> -				redirty_page_for_writepage(wbc, page);
> +			if (buffer_dirty(bh)) {
> +				if (!PageDirty(page))
> +					redirty_page_for_writepage(wbc, page);
> +				keep_towrite = true;
> +			}
>  			if (io->io_bio)
>  				ext4_io_submit(io);
>  			continue;
> @@ -500,6 +501,10 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  		nr_to_submit++;
>  	} while ((bh = bh->b_this_page) != head);
>
> +	/* Nothing to submit? Just unlock the page... */
> +	if (!nr_to_submit)
> +		goto unlock;
> +
>  	bh = head = page_buffers(page);
>
>  	/*
> @@ -550,6 +555,11 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  		}
>  	}
>
> +	if (keep_towrite)
> +		set_page_writeback_keepwrite(page);
> +	else
> +		set_page_writeback(page);
> +
>  	/* Now submit buffers to write */
>  	do {
>  		if (!buffer_async_write(bh))
> @@ -558,11 +568,7 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  				 bounce_page ? bounce_page : page, bh);
>  		nr_submitted++;
>  	} while ((bh = bh->b_this_page) != head);
> -
>  unlock:
>  	unlock_page(page);
> -	/* Nothing submitted - we have to end page writeback */
> -	if (!nr_submitted)
> -		end_page_writeback(page);
>  	return ret;
>  }
> --
> 2.35.3
>

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