linux-kernel - Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

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Date:   Sat, 28 Apr 2018 20:54:42 +0000
From:   "Kani, Toshi" <toshi.kani@....com>
To:     "joro@...tes.org" <joro@...tes.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "guohanjun@...wei.com" <guohanjun@...wei.com>,
        "wxf.wang@...ilicon.com" <wxf.wang@...ilicon.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "Hocko, Michal" <MHocko@...e.com>,
        "cpandya@...eaurora.org" <cpandya@...eaurora.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

On Sat, 2018-04-28 at 11:02 +0200, joro@...tes.org wrote:
> On Fri, Apr 27, 2018 at 02:31:51PM +0000, Kani, Toshi wrote:
> > So, we can add the step 2 on top of this patch.
> >  1. Clear pud/pmd entry.
> >  2. System wide TLB flush <-- TO BE ADDED BY NEW PATCH
> >  3. Free its underlining pmd/pte page.
> 
> This still lacks the page-table synchronization and will thus not fix
> the BUG_ON being triggered.

The BUG_ON issue is specific to PAE that it syncs at the pmd level.
x86/64 does not have this issue since it syncs at the pgd or p4d level.

> > We do not need to revert this patch.  We can make the above change I
> > mentioned.
> 
> Please note that we are not in the merge window anymore and that any fix
> needs to be simple and obviously correct.

Understood.  Changing the x86/32 sync point is risky.  So, I am going to
revert the free page handling for PAE.   

Thanks,
-Toshi