linux-kernel - Re: [PATCH] fs/pipe.c: simplify tmp

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Message-ID: <aznlwadluqn3hpkmoe5zhsrpsybsif5wagoouhmevjwndigado@k4lbztlu23wf>
Date: Fri, 7 Mar 2025 23:43:13 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 
	David Howells <dhowells@...hat.com>, Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/pipe.c: simplify tmp_page handling

On Fri, Mar 07, 2025 at 11:10:04PM +0100, Rasmus Villemoes wrote:
> Assigning the newly allocated page to pipe->tmp_page, only to
> unconditionally clear ->tmp_page a little later, seems somewhat odd.
> 
> It made sense prior to commit a194dfe6e6f6 ("pipe: Rearrange sequence
> in pipe_write() to preallocate slot"), when a user copy was done
> between the allocation and the buf->page = page assignment, and a
> failure there would then just leave the pipe's one-element page cache
> populated. Now, the same purpose is served by the page being inserted
> as a size-0 buffer, and the next write attempting to merge with that
> buffer.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  fs/pipe.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 4d0799e4e719..097400cce241 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -508,13 +508,14 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
>  			struct page *page = pipe->tmp_page;
>  			int copied;
>  
> -			if (!page) {
> +			if (page) {
> +				pipe->tmp_page = NULL;
> +			} else {
>  				page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
>  				if (unlikely(!page)) {
>  					ret = ret ? : -ENOMEM;
>  					break;
>  				}
> -				pipe->tmp_page = page;
>  			}
>  
>  			/* Allocate a slot in the ring in advance and attach an
> @@ -534,7 +535,6 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
>  				buf->flags = PIPE_BUF_FLAG_PACKET;
>  			else
>  				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;
> -			pipe->tmp_page = NULL;
>  
>  			copied = copy_page_from_iter(page, 0, PAGE_SIZE, from);
>  			if (unlikely(copied < PAGE_SIZE && iov_iter_count(from))) {

This bit got reworked in https://lore.kernel.org/all/20250303230409.452687-3-mjguzik@gmail.com/

I see it is in -next, merged from https://web.git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=vfs-6.15.pipe