linux-kernel - Re: [PATCH, RFC 7/9] thp: implement splitting pmd for huge zero page

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Message-ID: <20120817163346.GC10129@redhat.com>
Date:	Fri, 17 Aug 2012 18:33:46 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH, RFC 7/9] thp: implement splitting pmd for huge zero page

On Fri, Aug 17, 2012 at 11:12:33AM +0300, Kirill A. Shutemov wrote:
> I've used do_huge_pmd_wp_page_fallback() as template for my code.
> What's difference between these two code paths?
> Why is do_huge_pmd_wp_page_fallback() safe?

Good point. do_huge_pmd_wp_page_fallback works only on the current
"mm" so it doesn't need the splitting transition, but thinking twice
the split_huge_zero_page_pmd also works only on the local "mm" because
you're not really splitting the zero page there (you're not affecting
other mm). As long as you keep holding the page_table_lock of the "mm"
that you're altering your current version is safe.

I got mistaken because I'm very used to think at split huge page as
something that cannot relay on the page_table_lock, but this is a
simpler case that isn't splitting the "page" but only the "pmd" of a
single "mm", so you can safely relay on the mm->page_table_lock :).

> Looks resonable. I'll update it in next revision.

Thanks. Of course the function parameter comments to avoid unnecessary
calls of find_vma, weren't related to the above locking issues.
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