linux-kernel - Re: [lkp-robot] [btrfs] beeeccca9b: WARNING:at_mm/util.c:#kvmalloc

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Message-ID: <20170531091202.GA769@vader>
Date:   Wed, 31 May 2017 02:12:02 -0700
From:   Omar Sandoval <osandov@...ndov.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Vinnie Magro <vmagro@...com>, David Sterba <dsterba@...e.com>,
        Omar Sandoval <osandov@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, lkp@...org
Subject: Re: [lkp-robot] [btrfs]  beeeccca9b:
 WARNING:at_mm/util.c:#kvmalloc_node

On Wed, May 31, 2017 at 08:51:28AM +0200, Michal Hocko wrote:
> On Wed 31-05-17 14:30:33, kernel test robot wrote:
> > 
> > FYI, we noticed the following commit:
> > 
> > commit: beeeccca9bebcec386cc31c250cff8a06cf27034 ("btrfs: Use kvzalloc instead of kzalloc/vmalloc in alloc_bitmap")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> 
> I have intentionally skipped alloc_bitmap because it relies on GFP_NOFS.
> This doesn't work properly when falling back to vmalloc and that is what
> the warning reported here says. I believe the right approach is to check
> whether the GFP_NOFS is _really_ needed and document why if yes.
> Otherwise drop the NOFS part in one patch with the explanation and
> convert it to kvmalloc in a separate patch.

Unfortunately we really do need GFP_NOFS here, the free space tree is
modified while we are committing a fs transaction, sometimes in the
critical section when we block new operations from joining the
transaction. Looking at the comment in kvmalloc_node():

	/*
	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
	 * so the given set of flags has to be compatible.
	 */
	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);

has alloc_bitmap() always been broken by virtue of calling vmalloc()
with GFP_NOFS?