Re: !PageLocked BUG_ON hit in clear_page_dirty_for_io

[Chris Mason <clm@fb.com>]

On Tue, Dec 08, 2015 at 11:25:28PM -0500, Dave Jones wrote:
> Not sure if I've already reported this one, but I've been seeing this
> a lot this last couple days.
> 
> kernel BUG at mm/page-writeback.c:2654!
> invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN

We ended up discussing this in more detail on lkml, but I'll summarize
here.

There were two problems.  First lock_page() might not actually lock the
page in v4.4-rc4, it can bail out if a signal is pending.  This got
fixed just before v4.4-rc5, so if you were on rc4, upgrade asap.

Second, prepare_pages had a bug for single page writes:

>From f0be89af049857bcc537a53fe2a2fae080e7a5bd Mon Sep 17 00:00:00 2001
From: Chris Mason <clm@fb.com>
Date: Mon, 14 Dec 2015 15:40:44 -0800
Subject: [PATCH] Btrfs: check prepare_uptodate_page() error code earlier

prepare_pages() may end up calling prepare_uptodate_page() twice if our
write only spans a single page.  But if the first call returns an error,
our page will be unlocked and its not safe to call it again.

This bug goes all the way back to 2011, and it's not something commonly
hit.

While we're here, add a more explicit check for the page being truncated
away.  The bare lock_page() alone is protected only by good thoughts and
i_mutex, which we're sure to regret eventually.

Reported-by: Dave Jones <dsj@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
---
 fs/btrfs/file.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 72e7346..0f09526 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1291,7 +1291,8 @@ out:
  * on error we return an unlocked page and the error value
  * on success we return a locked page and 0
  */
-static int prepare_uptodate_page(struct page *page, u64 pos,
+static int prepare_uptodate_page(struct inode *inode,
+				 struct page *page, u64 pos,
 				 bool force_uptodate)
 {
 	int ret = 0;
@@ -1306,6 +1307,10 @@ static int prepare_uptodate_page(struct page *page, u64 pos,
 			unlock_page(page);
 			return -EIO;
 		}
+		if (page->mapping != inode->i_mapping) {
+			unlock_page(page);
+			return -EAGAIN;
+		}
 	}
 	return 0;
 }
@@ -1324,6 +1329,7 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 	int faili;
 
 	for (i = 0; i < num_pages; i++) {
+again:
 		pages[i] = find_or_create_page(inode->i_mapping, index + i,
 					       mask | __GFP_WRITE);
 		if (!pages[i]) {
@@ -1333,13 +1339,17 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 		}
 
 		if (i == 0)
-			err = prepare_uptodate_page(pages[i], pos,
+			err = prepare_uptodate_page(inode, pages[i], pos,
 						    force_uptodate);
-		if (i == num_pages - 1)
-			err = prepare_uptodate_page(pages[i],
+		if (!err && i == num_pages - 1)
+			err = prepare_uptodate_page(inode, pages[i],
 						    pos + write_bytes, false);
 		if (err) {
 			page_cache_release(pages[i]);
+			if (err == -EAGAIN) {
+				err = 0;
+				goto again;
+			}
 			faili = i - 1;
 			goto fail;
 		}
-- 
2.4.6