linux-kernel - Re: [PATCH] mprotect: drop overprotective lock_pte

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Message-ID: <20170208134518.GM25530@redhat.com>
Date:   Wed, 8 Feb 2017 14:45:18 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mprotect: drop overprotective lock_pte_protection()

On Wed, Feb 08, 2017 at 03:04:21PM +0300, Kirill A. Shutemov wrote:
> On Tue, Feb 07, 2017 at 01:44:54PM -0800, Andrew Morton wrote:
> > On Tue,  7 Feb 2017 17:33:47 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> > 
> > > lock_pte_protection() uses pmd_lock() to make sure that we have stable
> > > PTE page table before walking pte range.
> > > 
> > > That's not necessary. We only need to make sure that PTE page table is
> > > established. It cannot vanish under us as long as we hold mmap_sem at
> > > least for read.
> > > 
> > > And we already have helper for that -- pmd_trans_unstable().
> > 
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-mprotect-use-pmd_trans_unstable-instead-of-taking-the-pmd_lock.patch
> > already did this?
> 
> Right. Except, it doesn't drop unneeded pmd_trans_unstable(pmd) check after
> __split_huge_pmd().
> 
> Could you fold this part of my patch into Andrea's?

Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>

> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index f9c07f54dd62..e919e4613eab 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -177,8 +149,6 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>  		if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
>  			if (next - addr != HPAGE_PMD_SIZE) {
>  				__split_huge_pmd(vma, pmd, addr, false, NULL);
> -				if (pmd_trans_unstable(pmd))
> -					continue;
>  			} else {
>  				int nr_ptes = change_huge_pmd(vma, pmd, addr,
>  						newprot, prot_numa);

Yes this check was an harmless noop, but it's definitely good to clean
up this bit too after the other more important change that has a
positive runtime effect, or it could be a source of confusion to the
reader if left in there.

Thanks!
Andrea