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From:   Miklos Szeredi <mszeredi@redhat.com>
Date:   Tue, 16 Jun 2020 14:35:20 +0200
Message-ID: <CAOssrKdTTZEBEfwFm+3dYJ5XB9FsFAeF5g5ydeSo7ZPKkqjJXQ@mail.gmail.com>
Subject: Re: [fuse] 6b2fb79963: WARNING:at_fs/fuse/file.c:#tree_insert[fuse]
To:     Vasily Averin <vvs@virtuozzo.com>
Cc:     Maxim Patlasov <maximvp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jun 15, 2020 at 7:59 AM Vasily Averin <vvs@virtuozzo.com> wrote:
>
> On 6/15/20 3:50 AM, kernel test robot wrote:
> > FYI, we noticed the following commit (built with gcc-9):
> >
> > commit: 6b2fb79963fbed7db3ef850926d913518fd5c62f ("fuse: optimize writepages search")
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>
> > [ 1030.995703] ------------[ cut here ]------------
> > [ 1030.997563] WARNING: CPU: 1 PID: 17211 at fs/fuse/file.c:1728 tree_insert+0xab/0xc0 [fuse]
> > [ 1031.021943] RIP: 0010:tree_insert+0xab/0xc0 [fuse]
> > [ 1031.057802] Call Trace:
> > [ 1031.060015]  fuse_writepages_fill+0x5da/0x6a0 [fuse]
> > [ 1031.062536]  write_cache_pages+0x171/0x470
> > [ 1031.064933]  ? fuse_writepages+0x100/0x100 [fuse]
> > [ 1031.067419]  ? terminate_walk+0xd3/0xf0
> > [ 1031.069707]  ? _cond_resched+0x19/0x30
> > [ 1031.072140]  ? __kmalloc+0x274/0x280
> > [ 1031.074407]  fuse_writepages+0x8a/0x100 [fuse]
> > [ 1031.076599]  do_writepages+0x43/0xe0
>
> It is  WARN_ON(!wpa->ia.ap.num_pages);
> however tree_insert() was called from fuse_writepages_fill() with ap->num_pages = 0;
> In submitted PATCH RFC we have used
>
> +static int tree_insert(struct rb_root *root, struct fuse_req *ins_req)
> ...
> +       pgoff_t idx_to   = idx_from + (ins_req->num_pages ?
> +                               ins_req->num_pages - 1 : 0);
>
> Though committed patch version have
>
> +static void tree_insert(struct rb_root *root, struct fuse_writepage_args *wpa)
> ...
> +       pgoff_t idx_to = idx_from + wpa->ia.ap.num_pages - 1;
> ...
> +       WARN_ON(!wpa->ia.ap.num_pages);
>
> Miklos,
> do you have any objections if I return to our initial proposal?
> Am I missed something and it is not allowed now?

No objections, but you need to explain with a comment why that special
casing of num_pages == 0 is needed.  I don't understand it yet.

Thanks,
Miklos