linux-kernel - Re: Re: [fuse] 6b2fb79963: WARNING:at_fs/fuse/file.c:#tree

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Date:   Thu, 18 Jun 2020 13:29:55 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Miklos Szeredi <mszeredi@...hat.com>,
        Vasily Averin <vvs@...tuozzo.com>
Cc:     Maxim Patlasov <maximvp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Re: [fuse] 6b2fb79963:
 WARNING:at_fs/fuse/file.c:#tree_insert[fuse]

Hi,

On 16.06.2020 15:35, Miklos Szeredi wrote:
> On Mon, Jun 15, 2020 at 7:59 AM Vasily Averin <vvs@...tuozzo.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.

Currently here is already a bomb in mainline kernel.

This fuse_writepages_fill()->tree_insert() with num_pages == 0 corrupts fi->writepages tree.
Normally we have half-closed intervals in the tree:


         [5, 6)
        /      \
  [1, 3)        [7, 9)

But tree_insert(num_pages == 0) adds an abnormal interval with both ends included:

         [5, 5]
        /      \
  [1, 3)        [7, 9)


This may seriously confuse a reader iterating a tree, which does not know about special case (num_pages == 0).
As I see, nobody in the code cares about num_pages == 0, and there is no comment about that case.

This will fire soon or later. I think we should stop insert abnormal intervals (num_pages == 0) there.

Reasons fuse_writepages_fill() does use this (num_pages == 0)?!