linux-kernel - Re: [PATCH RFC] ashmem: Fix lockdep RECLAIM

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Date:   Wed, 7 Feb 2018 18:29:01 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     NeilBrown <neilb@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        "Theodore Ts'o" <tytso@....edu>, linux-fsdevel@...r.kernel.org,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH RFC] ashmem: Fix lockdep RECLAIM_FS false positive

On Wed, Feb 7, 2018 at 4:35 PM, NeilBrown <neilb@...e.com> wrote:
>> On Wed, Feb 7, 2018 at 8:58 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> [...]
>>>
>>>> Lockdep reports this issue when GFP_FS is infact set, and we enter
>>>> this path and acquire the lock. So lockdep seems to be doing the right
>>>> thing however by design it is reporting a false-positive.
>>>
>>> So I'm not seeing how its a false positive. fs/inode.c sets a different
>>> lock class per filesystem type. So recursing on an i_mutex within a
>>> filesystem does sound dodgy.
>>
>> But directory inodes and file inodes in the same filesystem share the
>> same lock class right?
>
> Not since v2.6.24
> Commit: 14358e6ddaed ("lockdep: annotate dir vs file i_mutex")
>
> You were using 4.9.60. so they should be separate....
>
> Maybe shmem_get_inode() needs to call unlock_new_inode() or just
> lockdep_annotate_inode_mutex_key() after inode_init_owner().
>
> Maybe inode_init_owner() should call lockdep_annotate_inode_mutex_key()
> directly.

Thanks for the ideas! I will test lockdep_annotate_inode_mutex_key
after inode_init_owner in shmem and let you know if the issue goes
away. It seems hugetlbfs does this too (I think for similar reasons).

- Joel