linux-kernel - Re: [PATCH v17 03/14] shmem: Implement splice-read

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Date:   Tue, 14 Mar 2023 21:50:12 +0000
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Daniel Golle <daniel@...rotopia.org>,
        Guenter Roeck <groeck7@...il.com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v17 03/14] shmem: Implement splice-read

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> But please at least stop doing the
> 
>      get_page(buf->page);
> 
> on the zero-page (which includes using no-op .get and .put functions
> in  zero_pipe_buf_ops().

I'll make the attached change.  It seems to work.

David
---
diff --git a/mm/shmem.c b/mm/shmem.c
index 3cbec1d56112..d9b60ab556fe 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2719,6 +2719,17 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	return retval ? retval : error;
 }
 
+static bool zero_pipe_buf_get(struct pipe_inode_info *pipe,
+			      struct pipe_buffer *buf)
+{
+	return true;
+}
+
+static void zero_pipe_buf_release(struct pipe_inode_info *pipe,
+				  struct pipe_buffer *buf)
+{
+}
+
 static bool zero_pipe_buf_try_steal(struct pipe_inode_info *pipe,
 				    struct pipe_buffer *buf)
 {
@@ -2726,9 +2737,9 @@ static bool zero_pipe_buf_try_steal(struct pipe_inode_info *pipe,
 }
 
 static const struct pipe_buf_operations zero_pipe_buf_ops = {
-	.release	= generic_pipe_buf_release,
+	.release	= zero_pipe_buf_release,
 	.try_steal	= zero_pipe_buf_try_steal,
-	.get		= generic_pipe_buf_get,
+	.get		= zero_pipe_buf_get,
 };
 
 static size_t splice_zeropage_into_pipe(struct pipe_inode_info *pipe,