[Devel] [PATCH RH8] mm/tcache: restore missing rcu_read_lock() in tcache_detach_page()

[Andrey Ryabinin]

On 10/2/20 5:13 PM, Evgenii Shatokhin wrote:
> Looks like rcu_read_lock() was lost in "out:" path of tcache_detach_page()
> when tcache was ported to VZ8. As a result, Syzkaller was able to hit
> the following warning:
> 
>   WARNING: bad unlock balance detected!
>   4.18.0-193.6.3.vz8.4.7.syz+debug #1 Tainted: G        W        ---------r-  -
>   -------------------------------------
>   vcmmd/926 is trying to release lock (rcu_read_lock) at:
>   [<ffffffff848ed2e0>] tcache_detach_page+0x530/0x750
>   but there are no more locks to release!
> 
>   other info that might help us debug this:
>   2 locks held by vcmmd/926:
>    #0: ffff888036331f30 (&mm->mmap_sem){++++}, at: __do_page_fault+0x157/0x550
>    #1: ffff8880567295f8 (&ei->i_mmap_sem){++++}, at: ext4_filemap_fault+0x82/0xc0 [ext4]
> 
>   stack backtrace:
>   CPU: 0 PID: 926 Comm: vcmmd ve: /
>                Tainted: G        W        ---------r-  - 4.18.0-193.6.3.vz8.4.7.syz+debug #1 4.7
>   Hardware name: Virtuozzo KVM, BIOS 1.11.0-2.vz7.2 04/01/2014
>   Call Trace:
>    dump_stack+0xd2/0x148
>    print_unlock_imbalance_bug.cold.40+0xc8/0xd4
>    lock_release+0x5e3/0x1360
>    tcache_detach_page+0x559/0x750
>    tcache_cleancache_get_page+0xe9/0x780
>    __cleancache_get_page+0x212/0x320
>    ext4_mpage_readpages+0x165d/0x1b90 [ext4]
>    ext4_readpages+0xd6/0x110 [ext4]
>    read_pages+0xff/0x5b0
>    __do_page_cache_readahead+0x3fc/0x5b0
>    filemap_fault+0x912/0x1b80
>    ext4_filemap_fault+0x8a/0xc0 [ext4]
>    __do_fault+0x110/0x410
>    do_fault+0x622/0x1010
>    __handle_mm_fault+0x980/0x1120
>    handle_mm_fault+0x17f/0x610
>    __do_page_fault+0x25d/0x550
>    do_page_fault+0x38/0x290
>    do_async_page_fault+0x5b/0xe0
>    async_page_fault+0x1e/0x30
> 
> Let us restore rcu_read_lock().
> 
> https://jira.sw.ru/browse/PSBM-120802
> Signed-off-by: Evgenii Shatokhin <eshatokhin at virtuozzo.com>

Reviewed-by: Andrey Ryabinin <aryabinin at virtuozzo.com>