linux-kernel - Re: __link_block_group uses GFP

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Message-ID: <CABxMtgeihqoUWA=aqEXU9GYrydUufgpbZ6v5+BRYCdj9kiSUwQ@mail.gmail.com>
Date:   Tue, 28 Mar 2017 11:16:35 +0300
From:   Denis Kirjanov <dkirjanov@...udlinux.com>
To:     dsterba@...e.cz, Denis Kirjanov <dkirjanov@...udlinux.com>,
        Jeff Mahoney <jeffm@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, chris.mason@...ionio.com
Subject: Re: __link_block_group uses GFP_KERNEL

On 3/27/17, David Sterba <dsterba@...e.cz> wrote:
> On Sat, Mar 25, 2017 at 09:48:28AM +0300, Denis Kirjanov wrote:
>> On 3/25/17, Jeff Mahoney <jeffm@...e.com> wrote:
>> > On 3/24/17 5:02 AM, Denis Kirjanov wrote:
>> >> Hi guys,
>> >>
>> >> Looks like that current code does GFP_KERNEL allocation inside
>> >> __link_block_group.
>> >> the function invokes kobject_add and internally creates sysfs files
>> >> with the GFP_KERNEL flag set.
>> >
>> > Yep, that's a bug.
>> >
>> >> But since do_chunk_alloc executes insides the btrfs transaction it's
>> >> not allowed to sleep.
>> >
>> > It's allowed to sleep but isn't allowed to do reclaim that involves
>> > file
>> > system writeback.  Michal Hocko's allocation context idea would fix
>> > this, but it's not there yet, so we'll need to defer the kobject_add
>> > until we can use GFP_KERNEL.
>>
>> Ok, I see. Can you point out to the initial patchset?
>
> https://lwn.net/Articles/716323/
>
> Fixing this properly is a lot of work so we might need to add a
> temporary workaround, as Jeff suggests, to move calling into sysfs to a
> later time.
>

Care to send a patch?
Or I can dig a bit.

Thanks!