On Tue, May 12 2009, Andrew Morton wrote:
> On Thu, 07 May 2009 15:37:36 +0200 Miklos Szeredi <miklos@xxxxxxxxxx> 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@xxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
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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