linux-kernel - Re: [PATCH] mm/shmem: Zero out unused vma fields in shmem_pseudo_vma

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Date:   Mon, 4 Jun 2018 14:32:36 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Josef Bacik <jbacik@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/shmem: Zero out unused vma fields in
 shmem_pseudo_vma_init()

On Thu, May 31, 2018 at 10:50:36PM +0000, Hugh Dickins wrote:
> On Thu, 31 May 2018, Kirill A. Shutemov wrote:
> 
> > shmem/tmpfs uses pseudo vma to allocate page with correct NUMA policy.
> > 
> > The pseudo vma doesn't have vm_page_prot set. We are going to encode
> > encryption KeyID in vm_page_prot. Having garbage there causes problems.
> > 
> > Zero out all unused fields in the pseudo vma.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> I won't go so far as to say NAK, but personally I much prefer that we
> document what fields actually get used, by initializing only those,
> rather than having such a blanket memset.

I recognize value of documentation here. But I still think leaving garbage
in the fields is not a great idea.

> 
> And you say "We are going to ...": so this should really be part of
> some future patchset, shouldn't it?

Yeah. It's for MKTME. I just try to push easy patches first.

> My opinion might be in the minority: you remind me of a similar
> request from Josef some while ago, Cc'ing him.
> 
> (I'm very ashamed, by the way, of shmem's pseudo-vma, I think it's
> horrid, and just reflects that shmem was an afterthought when NUMA
> mempolicies were designed.  Internally, we replaced alloc_pages_vma()
> throughout by alloc_pages_mpol(), which has no need for pseudo-vmas,
> and the advantage of dropping mmap_sem across the bulk of NUMA page
> migration. I shall be updating that work in coming months, and hope
> to upstream, but no promise from me on the timing - your need for
> vm_page_prot likely much sooner.)

I will try to look at how we can get alloc_pages_mpol() implemented.
(Although interleave bias is kinda confusing. I'll need to wrap my head
around the thing.)

-- 
 Kirill A. Shutemov