Re: Possible deadlock in fuse write path (Was: Re: [PATCH 0/4] Some more lock_page work..)

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Oct 15, 2020 at 11:16:06AM -0400, Vivek Goyal wrote:
> On Wed, Oct 14, 2020 at 07:44:16PM -0700, Linus Torvalds wrote:
> > On Wed, Oct 14, 2020 at 6:48 PM Qian Cai <cai@lca.pw> wrote:
> > >
> > > While on this topic, I just want to bring up a bug report that we are chasing an
> > > issue that a process is stuck in the loop of wait_on_page_bit_common() for more
> > > than 10 minutes before I gave up.
> > 
> > Judging by call trace, that looks like a deadlock rather than a missed wakeup.
> > 
> > The trace isn't reliable, but I find it suspicious that the call trace
> > just before the fault contains that
> > "iov_iter_copy_from_user_atomic()".
> > 
> > IOW, I think you're in fuse_fill_write_pages(), which has allocated
> > the page, locked it, and then it takes a page fault.
> > 
> > And the page fault waits on a page that is locked.
> > 
> > This is a classic deadlock.
> > 
> > The *intent* is that iov_iter_copy_from_user_atomic() returns zero,
> > and you retry without the page lock held.
> > 
> > HOWEVER.
> > 
> > That's not what fuse actually does. Fuse will do multiple pages, and
> > it will unlock only the _last_ page. It keeps the other pages locked,
> > and puts them in an array:
> > 
> >                 ap->pages[ap->num_pages] = page;
> > 
> > And after the iov_iter_copy_from_user_atomic() fails, it does that
> > "unlock" and repeat.
> > 
> > But while the _last_ page was unlocked, the *previous* pages are still
> > locked in that array. Deadlock.
> > 
> > I really don't think this has anything at all to do with page locking,
> > and everything to do with fuse_fill_write_pages() having a deadlock if
> > the source of data is a mmap of one of the pages it is trying to write
> > to (just with an offset, so that it's not the last page).
> > 
> > See a similar code sequence in generic_perform_write(), but notice how
> > that code only has *one* page that it locks, and never holds an array
> > of pages around over that iov_iter_fault_in_readable() thing.
> 
> Indeed. This is a deadlock in fuse. Thanks for the analysis. I can now
> trivially reproduce it with following program.

I am wondering how should I fix this issue. Is it enough that I drop
the page lock (but keep the reference) inside the loop. And once copying
from user space is done, acquire page locks for all pages (Attached
a patch below).

Or dropping page lock means that there are no guarantees that this
page did not get written back and removed from address space and
a new page has been placed at same offset. Does that mean I should
instead be looking up page cache again after copying from user
space is done. 

---
 fs/fuse/file.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

Index: redhat-linux/fs/fuse/file.c
===================================================================
--- redhat-linux.orig/fs/fuse/file.c	2020-10-15 14:11:23.464720742 -0400
+++ redhat-linux/fs/fuse/file.c	2020-10-15 15:23:17.716569698 -0400
@@ -1117,7 +1117,7 @@ static ssize_t fuse_fill_write_pages(str
 	struct fuse_conn *fc = get_fuse_conn(mapping->host);
 	unsigned offset = pos & (PAGE_SIZE - 1);
 	size_t count = 0;
-	int err;
+	int err, i;
 
 	ap->args.in_pages = true;
 	ap->descs[0].offset = offset;
@@ -1155,6 +1155,14 @@ static ssize_t fuse_fill_write_pages(str
 			goto again;
 		}
 
+		/*
+		 * Unlock page but retain reference count. Unlock is requied
+		 * otherwise it is possible that next loop iteration tries
+		 * to fault in page (source address) we have lock on and we
+		 * will deadlock. So drop lock but keep a reference on the
+		 * page. Re-acquire page lock after breaking out of the loop
+		 */
+		unlock_page(page);
 		err = 0;
 		ap->pages[ap->num_pages] = page;
 		ap->descs[ap->num_pages].length = tmp;
@@ -1171,7 +1179,15 @@ static ssize_t fuse_fill_write_pages(str
 	} while (iov_iter_count(ii) && count < fc->max_write &&
 		 ap->num_pages < max_pages && offset == 0);
 
-	return count > 0 ? count : err;
+	if (count <= 0)
+		return err;
+
+	for (i = 0; i < ap->num_pages; i++) {
+		/* Take page lock */
+		lock_page(ap->pages[i]);
+	}
+
+	return count;
 }
 
 static inline unsigned int fuse_wr_pages(loff_t pos, size_t len,