Re: [PATCH RFC] ashmem: Fix lockdep RECLAIM_FS false positive

[Joel Fernandes]

Hi Minchan,

On Tue, Feb 6, 2018 at 2:01 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
[...]
> On Mon, Feb 05, 2018 at 04:49:03PM -0800, Joel Fernandes wrote:
>> During invocation of ashmem shrinker under memory pressure, ashmem
>> calls into VFS code via vfs_fallocate. We however make sure we
>> don't enter it if the allocation was GFP_FS to prevent looping
>> into filesystem code. However lockdep doesn't know this and prints
>> a lockdep splat as below.
>>
>> This patch fixes the issue by releasing the reclaim_fs lock after
>> checking for GFP_FS but before calling into the VFS path, and
>> reacquiring it after so that lockdep can continue reporting any
>> reclaim issues later.
>
> At first glance, it looks reasonable. However, Couldn't we return
> just 0 in ashmem_shrink_count when the context is under FS?
>

We're already checking if GFP_FS in ashmem_shrink_scan and bailing out
though, did I miss something?

The problem is not that there is a deadlock that occurs, the problem
that even when we're not under FS, lockdep reports an issue that can't
happen. The fix is for the lockdep false positive that occurs.

thanks,

- Joel