linux-kernel - Re: __link_block_group uses GFP

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Message-ID: <20170327171817.GN4781@twin.jikos.cz>
Date:   Mon, 27 Mar 2017 19:18:17 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Denis Kirjanov <dkirjanov@...udlinux.com>
Cc:     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 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.