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

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Date:   Fri, 2 Jun 2017 09:28:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
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 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.
-- 
Michal Hocko
SUSE Labs