linux-kernel - Re: [PATCH] iomap: avoid unnecessary ifs_set_range

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Message-ID: <20250701184737.GA9991@frogsfrogsfrogs>
Date: Tue, 1 Jul 2025 11:47:37 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: alexjlzheng@...il.com
Cc: brauner@...nel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with
 locks

On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@...il.com wrote:
> From: Jinliang Zheng <alexjlzheng@...cent.com>
> 
> In the buffer write path, iomap_set_range_uptodate() is called every
> time iomap_end_write() is called. But if folio_test_uptodate() holds, we
> know that all blocks in this folio are already in the uptodate state, so
> there is no need to go deep into the critical section of state_lock to
> execute bitmap_set().
> 
> Although state_lock may not have significant lock contention due to
> folio lock, this patch at least reduces the number of instructions.
> 
> Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> ---
>  fs/iomap/buffered-io.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 3729391a18f3..fb4519158f3a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -71,6 +71,9 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
>  	unsigned long flags;
>  	bool uptodate = true;
>  
> +	if (folio_test_uptodate(folio))
> +		return;

Looks fine, but how exhaustively have you tested this with heavy IO
workloads?  I /think/ it's the case that folios always creep towards
ifs_is_fully_uptodate() == true state and once they've gotten there
never go back.  But folio state bugs are tricky to detect once they've
crept in.

--D

> +
>  	if (ifs) {
>  		spin_lock_irqsave(&ifs->state_lock, flags);
>  		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> -- 
> 2.49.0
> 
>