linux-kernel - Re: [patch 2/3] splice: implement default splice

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Date:	Wed, 13 May 2009 08:37:07 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Max Kellermann <max@...mpel.org>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/3] splice: implement default splice_read method

On Tue, May 12 2009, Andrew Morton wrote:
> On Thu, 07 May 2009 15:37:36 +0200 Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > +	for (i = 0; i < spd.nr_pages; i++) {
> > +		kunmap(pages[i]);
> 
> It is deadlockable if any thread of control holds more than a single
> kmap at a time.
> 
> Because there are a finite number of kmaps available, and if one is
> unavailable, kmap() waits for one to become free.  If the number of
> waiting threads equals the number of available slots, nobody makes any
> progress.

Good catch, that will not work reliably. I've applied the below.

commit 4f23122858a27ba97444b9b37a066d83edebd4c8
Author: Jens Axboe <jens.axboe@...cle.com>
Date:   Wed May 13 08:35:35 2009 +0200

    splice: fix repeated kmap()'s in default_file_splice_read()
    
    We cannot reliably map more than one page at the time, or we risk
    deadlocking. Just allocate the pages from low mem instead.
    
    Reported-by: Andrew Morton <akpm@...ux-foundation.org>
    Signed-off-by: Jens Axboe <jens.axboe@...cle.com>

diff --git a/fs/splice.c b/fs/splice.c
index eefd96b..c5e3c79 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -580,13 +580,13 @@ ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
 	for (i = 0; i < nr_pages && i < PIPE_BUFFERS && len; i++) {
 		struct page *page;
 
-		page = alloc_page(GFP_HIGHUSER);
+		page = alloc_page(GFP_USER);
 		error = -ENOMEM;
 		if (!page)
 			goto err;
 
 		this_len = min_t(size_t, len, PAGE_CACHE_SIZE - offset);
-		vec[i].iov_base = (void __user *) kmap(page);
+		vec[i].iov_base = (void __user *) page_address(page);
 		vec[i].iov_len = this_len;
 		pages[i] = page;
 		spd.nr_pages++;
@@ -604,7 +604,6 @@ ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
 
 	nr_freed = 0;
 	for (i = 0; i < spd.nr_pages; i++) {
-		kunmap(pages[i]);
 		this_len = min_t(size_t, vec[i].iov_len, res);
 		partial[i].offset = 0;
 		partial[i].len = this_len;
@@ -624,10 +623,9 @@ ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
 	return res;
 
 err:
-	for (i = 0; i < spd.nr_pages; i++) {
-		kunmap(pages[i]);
+	for (i = 0; i < spd.nr_pages; i++)
 		__free_page(pages[i]);
-	}
+
 	return error;
 }
 EXPORT_SYMBOL(default_file_splice_read);

-- 
Jens Axboe

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