Date:   Thu, 1 Dec 2022 16:51:52 +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 9/9] ext4: Remove ordered data support from
 ext4_writepage()

On 22/11/30 05:36PM, Jan Kara wrote:
> ext4_writepage() should not be called for ordered data anymore. Remove
> support for it from the function.
>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/ext4/inode.c | 116 ++++++------------------------------------------
>  1 file changed, 13 insertions(+), 103 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c131b611dabf..0c8e700265f1 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1642,12 +1642,6 @@ static void ext4_print_free_blocks(struct inode *inode)
>  	return;
>  }
>
> -static int ext4_bh_delay_or_unwritten(handle_t *handle, struct inode *inode,
> -				      struct buffer_head *bh)
> -{
> -	return (buffer_delay(bh) || buffer_unwritten(bh)) && buffer_dirty(bh);
> -}
> -
>  /*
>   * ext4_insert_delayed_block - adds a delayed block to the extents status
>   *                             tree, incrementing the reserved cluster/block
> @@ -1962,56 +1956,17 @@ static int __ext4_journalled_writepage(struct page *page,
>  }
>
>  /*
> - * Note that we don't need to start a transaction unless we're journaling data
> - * because we should have holes filled from ext4_page_mkwrite(). We even don't
> - * need to file the inode to the transaction's list in ordered mode because if
> - * we are writing back data added by write(), the inode is already there and if
> - * we are writing back data modified via mmap(), no one guarantees in which
> - * transaction the data will hit the disk. In case we are journaling data, we
> - * cannot start transaction directly because transaction start ranks above page
> - * lock so we have to do some magic.
> - *
> - * This function can get called via...
> - *   - ext4_writepages after taking page lock (have journal handle)
> - *   - journal_submit_inode_data_buffers (no journal handle)
> - *   - shrink_page_list via the kswapd/direct reclaim (no journal handle)
> - *   - grab_page_cache when doing write_begin (have journal handle)
> - *
> - * We don't do any block allocation in this function. If we have page with
> - * multiple blocks we need to write those buffer_heads that are mapped. This
> - * is important for mmaped based write. So if we do with blocksize 1K
> - * truncate(f, 1024);
> - * a = mmap(f, 0, 4096);
> - * a[0] = 'a';
> - * truncate(f, 4096);
> - * we have in the page first buffer_head mapped via page_mkwrite call back
> - * but other buffer_heads would be unmapped but dirty (dirty done via the
> - * do_wp_page). So writepage should write the first block. If we modify
> - * the mmap area beyond 1024 we will again get a page_fault and the
> - * page_mkwrite callback will do the block allocation and mark the
> - * buffer_heads mapped.
> - *
> - * We redirty the page if we have any buffer_heads that is either delay or
> - * unwritten in the page.
> - *
> - * We can get recursively called as show below.
> - *
> - *	ext4_writepage() -> kmalloc() -> __alloc_pages() -> page_launder() ->
> - *		ext4_writepage()
> - *
> - * But since we don't do any block allocation we should not deadlock.
> - * Page also have the dirty flag cleared so we don't get recurive page_lock.
> + * This function is now used only when journaling data. We cannot start
> + * transaction directly because transaction start ranks above page lock so we
> + * have to do some magic.
>   */
>  static int ext4_writepage(struct page *page,
>  			  struct writeback_control *wbc)
>  {
>  	struct folio *folio = page_folio(page);
> -	int ret = 0;
>  	loff_t size;
>  	unsigned int len;
> -	struct buffer_head *page_bufs = NULL;
>  	struct inode *inode = page->mapping->host;
> -	struct ext4_io_submit io_submit;
>
>  	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb)))) {
>  		folio_invalidate(folio, 0, folio_size(folio));
> @@ -2036,60 +1991,16 @@ static int ext4_writepage(struct page *page,
>  		return 0;
>  	}
>
> -	page_bufs = page_buffers(page);
> -	/*
> -	 * We cannot do block allocation or other extent handling in this
> -	 * function. If there are buffers needing that, we have to redirty
> -	 * the page. But we may reach here when we do a journal commit via
> -	 * journal_submit_inode_data_buffers() and in that case we must write
> -	 * allocated buffers to achieve data=ordered mode guarantees.

Maybe this description can go above mpage_prepare_extent_to_map
for can_map = 0 case.

Looks good to me. Feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>



> -	 *
> -	 * Also, if there is only one buffer per page (the fs block
> -	 * size == the page size), if one buffer needs block
> -	 * allocation or needs to modify the extent tree to clear the
> -	 * unwritten flag, we know that the page can't be written at
> -	 * all, so we might as well refuse the write immediately.
> -	 * Unfortunately if the block size != page size, we can't as
> -	 * easily detect this case using ext4_walk_page_buffers(), but
> -	 * for the extremely common case, this is an optimization that
> -	 * skips a useless round trip through ext4_bio_write_page().
> -	 */
> -	if (ext4_walk_page_buffers(NULL, inode, page_bufs, 0, len, NULL,
> -				   ext4_bh_delay_or_unwritten)) {
> -		redirty_page_for_writepage(wbc, page);
> -		if ((current->flags & PF_MEMALLOC) ||
> -		    (inode->i_sb->s_blocksize == PAGE_SIZE)) {
> -			/*
> -			 * For memory cleaning there's no point in writing only
> -			 * some buffers. So just bail out. Warn if we came here
> -			 * from direct reclaim.
> -			 */
> -			WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD))
> -							== PF_MEMALLOC);
> -			unlock_page(page);
> -			return 0;
> -		}
> -	}
> -
> -	if (PageChecked(page) && ext4_should_journal_data(inode))
> -		/*
> -		 * It's mmapped pagecache.  Add buffers and journal it.  There
> -		 * doesn't seem much point in redirtying the page here.
> -		 */
> -		return __ext4_journalled_writepage(page, len);
> -
> -	ext4_io_submit_init(&io_submit, wbc);
> -	io_submit.io_end = ext4_init_io_end(inode, GFP_NOFS);
> -	if (!io_submit.io_end) {
> -		redirty_page_for_writepage(wbc, page);
> +	WARN_ON_ONCE(!ext4_should_journal_data(inode));
> +	if (!PageChecked(page)) {
>  		unlock_page(page);
> -		return -ENOMEM;
> +		return 0;
>  	}
> -	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);
> -	return ret;
> +	/*
> +	 * It's mmapped pagecache.  Add buffers and journal it.  There
> +	 * doesn't seem much point in redirtying the page here.
> +	 */
> +	return __ext4_journalled_writepage(page, len);
>  }
>
>  static int mpage_submit_page(struct mpage_da_data *mpd, struct page *page)
> @@ -3142,9 +3053,8 @@ static int ext4_da_write_end(struct file *file,
>  	 * i_disksize since writeback will push i_disksize upto i_size
>  	 * eventually. If the end of the current write is > i_size and
>  	 * inside an allocated block (ext4_da_should_update_i_disksize()
> -	 * check), we need to update i_disksize here as neither
> -	 * ext4_writepage() nor certain ext4_writepages() paths not
> -	 * allocating blocks update i_disksize.
> +	 * check), we need to update i_disksize here as ext4_writepages() need
> +	 * not do it in this case.
>  	 *
>  	 * Note that we defer inode dirtying to generic_write_end() /
>  	 * ext4_da_write_inline_data_end().
> --
> 2.35.3
>

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