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

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Date:   Thu, 08 Feb 2018 11:35:43 +1100
From:   NeilBrown <neilb@...e.com>
To:     Joel Fernandes <joelaf@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 07 2018, Joel Fernandes wrote:

> Hi Peter,
>
> 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.

NeilBrown

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