linux-kernel - Re: [PATCH v3 03/14] mm: use pmd lock instead of racy checks in zap_pmd

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Date:   Tue, 07 Feb 2017 19:25:30 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Zi Yan <zi.yan@...t.com>, mgorman@...hsingularity.net,
        riel@...hat.com
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org,
        minchan@...nel.org, vbabka@...e.cz, n-horiguchi@...jp.nec.com,
        khandual@...ux.vnet.ibm.com, zi.yan@...rutgers.edu,
        Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v3 03/14] mm: use pmd lock instead of racy checks in zap_pmd_range()

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Sun, Feb 05, 2017 at 11:12:41AM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>> 
>> Originally, zap_pmd_range() checks pmd value without taking pmd lock.
>> This can cause pmd_protnone entry not being freed.
>> 
>> Because there are two steps in changing a pmd entry to a pmd_protnone
>> entry. First, the pmd entry is cleared to a pmd_none entry, then,
>> the pmd_none entry is changed into a pmd_protnone entry.
>> The racy check, even with barrier, might only see the pmd_none entry
>> in zap_pmd_range(), thus, the mapping is neither split nor zapped.
>
> That's definately a good catch.
>
> But I don't agree with the solution. Taking pmd lock on each
> zap_pmd_range() is a significant hit by scalability of the code path.
> Yes, split ptl lock helps, but it would be nice to avoid the lock in first
> place.
>
> Can we fix change_huge_pmd() instead? Is there a reason why we cannot
> setup the pmd_protnone() atomically?
>
> Mel? Rik?
>

I am also trying to fixup the usage of set_pte_at on ptes that are
valid/present (that this autonuma ptes). I guess what we are missing is a
variant of pte update routines that can atomically update a pte without
clearing it and that also doesn't do a tlb flush ?

-aneesh