linux-kernel - Re: [PATCH 4/7] jbd2: remove unneeded kmap for jh_in->b_frozen_data in jbd2_journal_write_metadata

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Message-ID: <20240730133248.k4ttccw3gla6xgoy@quack3>
Date: Tue, 30 Jul 2024 15:32:48 +0200
From: Jan Kara <jack@...e.cz>
To: Kemeng Shi <shikemeng@...weicloud.com>
Cc: tytso@....edu, jack@...e.com, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] jbd2: remove unneeded kmap for jh_in->b_frozen_data
 in jbd2_journal_write_metadata_buffer

On Tue 30-07-24 19:33:32, Kemeng Shi wrote:
> Remove kmap for page of b_frozen_data from jbd2_alloc() which always
> provides an address from the direct kernel mapping.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>

I don't think this is really a win. On majority of installations kmap is a
noop anyway and for the remainder kmap_local() is cheap. And the
readability of the code is just worse with this.

								Honza

> ---
>  fs/jbd2/journal.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 312c7575b54f..9c1ffb0dc740 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -352,12 +352,13 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction,
>  		done_copy_out = 1;
>  		new_folio = virt_to_folio(jh_in->b_frozen_data);
>  		new_offset = offset_in_folio(new_folio, jh_in->b_frozen_data);
> +		mapped_data = jh_in->b_frozen_data;
>  	} else {
>  		new_folio = bh_in->b_folio;
>  		new_offset = offset_in_folio(new_folio, bh_in->b_data);
> +		mapped_data = kmap_local_folio(new_folio, new_offset);
>  	}
>  
> -	mapped_data = kmap_local_folio(new_folio, new_offset);
>  	/*
>  	 * Fire data frozen trigger if data already wasn't frozen.  Do this
>  	 * before checking for escaping, as the trigger may modify the magic
> @@ -373,7 +374,8 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction,
>  	 */
>  	if (*((__be32 *)mapped_data) == cpu_to_be32(JBD2_MAGIC_NUMBER))
>  		do_escape = 1;
> -	kunmap_local(mapped_data);
> +	if (!jh_in->b_frozen_data)
> +		kunmap_local(mapped_data);
>  
>  	/*
>  	 * Do we need to do a data copy?
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR