linux-ext4 - Re: [PATCH v3 08/10] ext4: reserved credits for one extent during the folio writeback

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Message-ID: <3lhwbxlfcqt5ou3z2xzo7o7zdvpmgldju33cd2wqnvsszfhnaf@i2qkwhrja7be>
Date: Wed, 2 Jul 2025 16:34:24 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz, 
	ojaswin@...ux.ibm.com, sashal@...nel.org, yi.zhang@...wei.com, libaokun1@...wei.com, 
	yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 08/10] ext4: reserved credits for one extent during
 the folio writeback

On Tue 01-07-25 21:06:33, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> After ext4 supports large folios, reserving journal credits for one
> maximum-ordered folio based on the worst case cenario during the
> writeback process can easily exceed the maximum transaction credits.
> Additionally, reserving journal credits for one page is also no
> longer appropriate.
> 
> Currently, the folio writeback process can either extend the journal
> credits or initiate a new transaction if the currently reserved journal
> credits are insufficient. Therefore, it can be modified to reserve
> credits for only one extent at the outset. In most cases involving
> continuous mapping, these credits are generally adequate, and we may
> only need to perform some basic credit expansion. However, in extreme
> cases where the block size and folio size differ significantly, or when
> the folios are sufficiently discontinuous, it may be necessary to
> restart a new transaction and resubmit the folios.
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Looks good. Feel free to add:

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

								Honza

> ---
>  fs/ext4/inode.c | 25 ++++++++-----------------
>  1 file changed, 8 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 3230734a3014..ceaede80d791 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2546,21 +2546,6 @@ static int mpage_map_and_submit_extent(handle_t *handle,
>  	return err;
>  }
>  
> -/*
> - * Calculate the total number of credits to reserve for one writepages
> - * iteration. This is called from ext4_writepages(). We map an extent of
> - * up to MAX_WRITEPAGES_EXTENT_LEN blocks and then we go on and finish mapping
> - * the last partial page. So in total we can map MAX_WRITEPAGES_EXTENT_LEN +
> - * bpp - 1 blocks in bpp different extents.
> - */
> -static int ext4_da_writepages_trans_blocks(struct inode *inode)
> -{
> -	int bpp = ext4_journal_blocks_per_folio(inode);
> -
> -	return ext4_meta_trans_blocks(inode,
> -				MAX_WRITEPAGES_EXTENT_LEN + bpp - 1, bpp);
> -}
> -
>  static int ext4_journal_folio_buffers(handle_t *handle, struct folio *folio,
>  				     size_t len)
>  {
> @@ -2917,8 +2902,14 @@ static int ext4_do_writepages(struct mpage_da_data *mpd)
>  		 * not supported by delalloc.
>  		 */
>  		BUG_ON(ext4_should_journal_data(inode));
> -		needed_blocks = ext4_da_writepages_trans_blocks(inode);
> -
> +		/*
> +		 * Calculate the number of credits needed to reserve for one
> +		 * extent of up to MAX_WRITEPAGES_EXTENT_LEN blocks. It will
> +		 * attempt to extend the transaction or start a new iteration
> +		 * if the reserved credits are insufficient.
> +		 */
> +		needed_blocks = ext4_chunk_trans_blocks(inode,
> +						MAX_WRITEPAGES_EXTENT_LEN);
>  		/* start a new transaction */
>  		handle = ext4_journal_start_with_reserve(inode,
>  				EXT4_HT_WRITE_PAGE, needed_blocks, rsv_blocks);
> -- 
> 2.46.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR