Re: bad page state in 3.13-rc4

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Fri, Dec 20, 2013 at 5:02 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Why not just get rid of the idiotic get_user_pages() crap then?
> Something like the attached patch?
>
> Totally untested, but at least it makes *some* amount of sense.

Ok, that can't work, since the ring_pages[] allocation happens later.
So that part needs to be moved up, and it needs to initialize
'nr_pages'.

So here's the same patch, but with stuff moved around a bit, and the
"oops, couldn't create page" part fixed.

Bit it's still totally and entirely untested.

                   Linus

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 fs/aio.c | 54 +++++++++++++++++++++---------------------------------
 1 file changed, 21 insertions(+), 33 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 6efb7f6cb22e..3e857e98fb87 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -347,6 +347,20 @@ static int aio_setup_ring(struct kioctx *ctx)
 		return -EAGAIN;
 	}
 
+	ctx->aio_ring_file = file;
+	nr_events = (PAGE_SIZE * nr_pages - sizeof(struct aio_ring))
+			/ sizeof(struct io_event);
+
+	ctx->ring_pages = ctx->internal_pages;
+	if (nr_pages > AIO_RING_PAGES) {
+		ctx->ring_pages = kcalloc(nr_pages, sizeof(struct page *),
+					  GFP_KERNEL);
+		if (!ctx->ring_pages) {
+			put_aio_ring_file(ctx);
+			return -ENOMEM;
+		}
+	}
+
 	for (i = 0; i < nr_pages; i++) {
 		struct page *page;
 		page = find_or_create_page(file->f_inode->i_mapping,
@@ -358,19 +372,14 @@ static int aio_setup_ring(struct kioctx *ctx)
 		SetPageUptodate(page);
 		SetPageDirty(page);
 		unlock_page(page);
+
+		ctx->ring_pages[i] = page;
 	}
-	ctx->aio_ring_file = file;
-	nr_events = (PAGE_SIZE * nr_pages - sizeof(struct aio_ring))
-			/ sizeof(struct io_event);
+	ctx->nr_pages = i;
 
-	ctx->ring_pages = ctx->internal_pages;
-	if (nr_pages > AIO_RING_PAGES) {
-		ctx->ring_pages = kcalloc(nr_pages, sizeof(struct page *),
-					  GFP_KERNEL);
-		if (!ctx->ring_pages) {
-			put_aio_ring_file(ctx);
-			return -ENOMEM;
-		}
+	if (unlikely(i != nr_pages)) {
+		aio_free_ring(ctx);
+		return -EAGAIN;
 	}
 
 	ctx->mmap_size = nr_pages * PAGE_SIZE;
@@ -380,8 +389,8 @@ static int aio_setup_ring(struct kioctx *ctx)
 	ctx->mmap_base = do_mmap_pgoff(ctx->aio_ring_file, 0, ctx->mmap_size,
 				       PROT_READ | PROT_WRITE,
 				       MAP_SHARED | MAP_POPULATE, 0, &populate);
+	up_write(&mm->mmap_sem);
 	if (IS_ERR((void *)ctx->mmap_base)) {
-		up_write(&mm->mmap_sem);
 		ctx->mmap_size = 0;
 		aio_free_ring(ctx);
 		return -EAGAIN;
@@ -389,27 +398,6 @@ static int aio_setup_ring(struct kioctx *ctx)
 
 	pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
 
-	/* We must do this while still holding mmap_sem for write, as we
-	 * need to be protected against userspace attempting to mremap()
-	 * or munmap() the ring buffer.
-	 */
-	ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
-				       1, 0, ctx->ring_pages, NULL);
-
-	/* Dropping the reference here is safe as the page cache will hold
-	 * onto the pages for us.  It is also required so that page migration
-	 * can unmap the pages and get the right reference count.
-	 */
-	for (i = 0; i < ctx->nr_pages; i++)
-		put_page(ctx->ring_pages[i]);
-
-	up_write(&mm->mmap_sem);
-
-	if (unlikely(ctx->nr_pages != nr_pages)) {
-		aio_free_ring(ctx);
-		return -EAGAIN;
-	}
-
 	ctx->user_id = ctx->mmap_base;
 	ctx->nr_events = nr_events; /* trusted copy */