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

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Date:	Wed, 1 Oct 2014 15:42:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Hugh Dickins <hughd@...gle.com>, 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>
Subject: Re: pipe/page fault oddness.

On Wed, Oct 1, 2014 at 3:08 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> I've tried this patch on the same configuration that was triggering
> the VM_BUG_ON that Hugh mentioned previously. Surprisingly enough it
> ran fine for ~20 minutes before exploding with:

Well, that's somewhat encouraging. I didn't expect it to be perfect.

That said, "ran fine" isn't necessarily the same thing as "worked".
Who knows how buggy it was without showing overt symptoms until the
BUG_ON() triggered. But hey, I'll be optimistic.

> [ 2781.566206] kernel BUG at mm/huge_memory.c:1293!

So that's

        BUG_ON(is_huge_zero_page(page));

and the reason is trivial: the old code used to have a magical special
case for the zero-page hugepage (see change_huge_pmd()) and I got rid
of that (because now it's just about setting protections, and the
zero-page hugepage is in no way special.

So I think the solution is equally trivial: just accept that the
zero-page can happen, and ignore it (just un-numa it).

Appended is a incremental diff on top of the previous one. Even less
tested than the last case, but I think you get the idea if it doesn't
work as-is.

             Linus

View attachment "patch.diff" of type "text/plain" (806 bytes)