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@xxxxxxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxx/
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
