linux-kernel - Re: pipe/page fault oddness.

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Date:	Thu, 2 Oct 2014 08:57:32 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Dave Jones <davej@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: pipe/page fault oddness.

On Thu, Oct 2, 2014 at 1:47 AM, Hugh Dickins <hughd@...gle.com> wrote:
>
> I hesitate to admit, I still don't see it: please illuminate further.

No, your'e looking at what I was looking.

> We're talking about the loop in __split_huge_page_map(), where it does

Yes.

>                         entry = mk_pte(page + i, vma->vm_page_prot);
>                         entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>                         if (!pmd_write(*pmd))
>                                 entry = pte_wrprotect(entry);
>                         if (!pmd_young(*pmd))
>                                 entry = pte_mkold(entry);
>                         if (pmd_numa(*pmd))
>                                 entry = pte_mknuma(entry);
>
> , right?  I only see that adding _PAGE_NUMA to _PAGE_PROTNONE if
> pmd_numa(*pmd): but that would mean we had already gone wrong, setting
> pmd_numa in a PROT_NONE vma, which task_numa_work takes care not to do;
> or have mprotected an area to PROT_NONE without doing the pmd_mknonnuma.

Fair enough. Except this code has no locking that I see, so if we
*ever* see that numa entry in the pmd while walking the page tables in
vmscan, we're basically screwed.

> Or are you noticing a deficiency in the pmd locking?  I have not
> worked my way through that, so cannot guarantee it, but please
> point me to the weakness where you see it.

So I don't see any locking at all wrt mprotect (or new mmap). That's
kind of the whole point for page-out - it bypasses all the normal VM
locks, and only uses the last pte locking.

So the whole use of vma->vm_page_prot here is a bit scary. That gets
modified outside of the page table locks. So how do you know it's not
already PROT_NONE, but mprotect just hasn't gotten to actually take
the page table locks yet?

I dunno. It all makes me just very nervous. The whole "numa bit is
separate from the protections, has different locking, and is just
oddly and subtly different" is really what I fundamentally object to.
And it seems so _unnecessary_. All this odd complexity for no actual
gain - just extra code, and extra room for subtle bugs. Which is
exactly why I hate that magic NUMA bit so much.

                  Linus
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