Re: [PATCH] iomap: avoid unnecessary ifs_set_range_uptodate() with locks

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jul 01, 2025 at 10:48:47PM +0800, alexjlzheng@gmail.com wrote:
> From: Jinliang Zheng <alexjlzheng@tencent.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.

That means the uptodate bitmap is stale in that case.  That would
only matter if we could clear the folio uptodate bit and still
expect the page content to survive.  Which sounds dubious and I could
not find anything relevant grepping the tree, but I'm adding the
linux-mm list just in case.

> 
> Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.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;
> +
>  	if (ifs) {
>  		spin_lock_irqsave(&ifs->state_lock, flags);
>  		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> -- 
> 2.49.0
> 
> 
---end quoted text---