linux-kernel - Re: [PATCH] splice: Clean up direct_splice

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Message-ID: <3114484.1676468851@warthog.procyon.org.uk>
Date:   Wed, 15 Feb 2023 13:47:31 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>, 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,
        syzbot+a440341a59e3b7142895@...kaller.appspotmail.com,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH] splice: Clean up direct_splice_read() a bit

I forgot to commit the cast change too.  Try the attached instead.

David
---
splice: Clean up direct_splice_read() a bit

Do a couple of cleanups to direct_splice_read():

 (1) Cast to struct page **, not void *.

 (2) Simplify the calculation of the number of pages to keep/reclaim in
     direct_splice_read().

Suggested-by: Christoph Hellwig <hch@...radead.org>
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Jens Axboe <axboe@...nel.dk>
cc: Christoph Hellwig <hch@....de>
cc: Al Viro <viro@...iv.linux.org.uk>
cc: David Hildenbrand <david@...hat.com>
cc: John Hubbard <jhubbard@...dia.com>
cc: linux-mm@...ck.org
cc: linux-block@...r.kernel.org
cc: linux-fsdevel@...r.kernel.org
---

diff --git a/fs/splice.c b/fs/splice.c
index 9e798c901087..e97f9aa30717 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -295,7 +295,7 @@ ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	struct kiocb kiocb;
 	struct page **pages;
 	ssize_t ret;
-	size_t used, npages, chunk, remain, reclaim;
+	size_t used, npages, chunk, remain, keep = 0;
 	int i;
 
 	/* Work out how much data we can actually add into the pipe */
@@ -309,7 +309,7 @@ ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	if (!bv)
 		return -ENOMEM;
 
-	pages = (void *)(bv + npages);
+	pages = (struct page **)(bv + npages);
 	npages = alloc_pages_bulk_array(GFP_USER, npages, pages);
 	if (!npages) {
 		kfree(bv);
@@ -332,11 +332,8 @@ ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	kiocb.ki_pos = *ppos;
 	ret = call_read_iter(in, &kiocb, &to);
 
-	reclaim = npages * PAGE_SIZE;
-	remain = 0;
 	if (ret > 0) {
-		reclaim -= ret;
-		remain = ret;
+		keep = DIV_ROUND_UP(ret, PAGE_SIZE);
 		*ppos = kiocb.ki_pos;
 		file_accessed(in);
 	} else if (ret < 0) {
@@ -349,14 +346,12 @@ ssize_t direct_splice_read(struct file *in, loff_t *ppos,
 	}
 
 	/* Free any pages that didn't get touched at all. */
-	reclaim /= PAGE_SIZE;
-	if (reclaim) {
-		npages -= reclaim;
-		release_pages(pages + npages, reclaim);
-	}
+	if (keep < npages)
+		release_pages(pages + keep, npages - keep);
 
 	/* Push the remaining pages into the pipe. */
-	for (i = 0; i < npages; i++) {
+	remain = ret;
+	for (i = 0; i < keep; i++) {
 		struct pipe_buffer *buf = pipe_head_buf(pipe);
 
 		chunk = min_t(size_t, remain, PAGE_SIZE);