linux-kernel - Re: [PATCH 1/9] mm: introduce kv[mz]alloc helpers

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Date:   Fri, 2 Jun 2017 07:40:12 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 1/9] mm: introduce kv[mz]alloc helpers

On Fri, Jun 02, 2017 at 09:28:56AM +0200, Michal Hocko wrote:
> On Fri 02-06-17 07:17:22, Sasha Levin wrote:
> > On Mon, Mar 06, 2017 at 11:30:24AM +0100, Michal Hocko wrote:
> > > +void *kvmalloc_node(size_t size, gfp_t flags, int node)
> > > +{
> > > +	gfp_t kmalloc_flags = flags;
> > > +	void *ret;
> > > +
> > > +	/*
> > > +	 * 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);
> > 
> > Hm, there are quite a few locations in the kernel that do something like:
> > 
> > 	__vmalloc(len, GFP_NOFS, PAGE_KERNEL);
> > 
> > According to your patch, vmalloc can't really do GFP_NOFS, right?
> 
> Yes. It is quite likely that they will just work because the hardcoded
> GFP_KERNEL inside the vmalloc path is in unlikely paths (page table
> allocations for example) but yes they are broken. I didn't convert some
> places which opencode the kvmalloc with GFP_NOFS because I strongly
> _believe_ that the GFP_NOFS should be revisited, checked whether it is
> needed, documented if so and then memalloc_nofs__{save,restore} be used
> for the scope which is reclaim recursion unsafe. This would turn all
> those vmalloc users to the default GFP_KERNEL and still do the right
> thing.

While you haven't converted those paths, other folks have picked up
on that:

	commit beeeccca9bebcec386cc31c250cff8a06cf27034
	Author: Vinnie Magro <vmagro@...com>
	Date:   Thu May 25 12:18:02 2017 -0700

	    btrfs: Use kvzalloc instead of kzalloc/vmalloc in alloc_bitmap
	[...]

Maybe we should make kvmalloc_node() fail non-GFP_KERNEL allocations
rather than just warn on them to make this error more evident? I'm not
sure how these warnings were missed during testing.

-- 

Thanks,
Sasha