Re: [PATCH 1/2] fuse: Fix race in fuse_writepage_in_flight()

[Miklos Szeredi]

On Thu, Jan 10, 2019 at 11:48 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> Hi, Miklos,
>
> any comments about this?

Is there a reproducer?  ISTR that fsx-linux with mmaps enabled was
good for stressing the writeback_cache code.

Thanks,
Miklos

>
> On 26.11.2018 12:46, Kirill Tkhai wrote:
> > Checking for FR_PENDING in fuse_writepage_in_flight() is racy.
> > It does not guarantee the first request in misc.write.next list
> > is not in userspace, since there we take fc->lock, while
> > fuse_dev_do_read() takes fiq->waitq.lock:
> >
> > fuse_dev_read()                           fuse_writepage_in_flight()
> >                                             test_bit(FR_PENDING)
> >   clear_bit(FR_PENDING)
> > handle old_req->pages[0] in userspace
> >                                             copy_highpage(old_req->pages[0], page)
> >                                             ^^^^^
> >                                             userspace never sees this pages
> >
> > The only reliable way to determ, whether we are able to replace
> > old_req's page, is to completely skip the first request in the list.
> > This patch makes the function to do that.
> >
> > Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > ---
> >  fs/fuse/file.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index b52f9baaa3e7..c6650c68b31a 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -1740,6 +1740,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
> >  {
> >       struct fuse_conn *fc = get_fuse_conn(new_req->inode);
> >       struct fuse_inode *fi = get_fuse_inode(new_req->inode);
> > +     struct fuse_req *first_req;
> >       struct fuse_req *tmp;
> >       struct fuse_req *old_req;
> >       bool found = false;
> > @@ -1764,6 +1765,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
> >       }
> >
> >       new_req->num_pages = 1;
> > +     first_req = old_req;
> >       for (tmp = old_req; tmp != NULL; tmp = tmp->misc.write.next) {
> >               BUG_ON(tmp->inode != new_req->inode);
> >               curr_index = tmp->misc.write.in.offset >> PAGE_SHIFT;
> > @@ -1773,7 +1775,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
> >               }
> >       }
> >
> > -     if (old_req->num_pages == 1 && test_bit(FR_PENDING, &old_req->flags)) {
> > +     if (old_req->num_pages == 1 && old_req != first_req) {
> >               struct backing_dev_info *bdi = inode_to_bdi(page->mapping->host);
> >
> >               copy_highpage(old_req->pages[0], page);
> >