linux-kernel - Re: [PATCH v3 0/3] fix free pmd/pte page handlings on x86

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Date:   Mon, 25 Jun 2018 19:53:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kani, Toshi" <toshi.kani@....com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "cpandya@...eaurora.org" <cpandya@...eaurora.org>
Subject: Re: [PATCH v3 0/3] fix free pmd/pte page handlings on x86

On Mon 25-06-18 14:56:26, Kani Toshimitsu wrote:
> On Sun, 2018-06-24 at 15:19 +0200, Thomas Gleixner wrote:
> > On Wed, 16 May 2018, Toshi Kani wrote:
> > 
> > > This series fixes two issues in the x86 ioremap free page handlings
> > > for pud/pmd mappings.
> > > 
> > > Patch 01 fixes BUG_ON on x86-PAE reported by Joerg.  It disables
> > > the free page handling on x86-PAE.
> > > 
> > > Patch 02-03 fixes a possible issue with speculation which can cause
> > > stale page-directory cache.
> > >  - Patch 02 is from Chintan's v9 01/04 patch [1], which adds a new arg
> > >    'addr', with my merge change to patch 01.
> > >  - Patch 03 adds a TLB purge (INVLPG) to purge page-structure caches
> > >    that may be cached by speculation.  See the patch descriptions for
> > >    more detal.
> > 
> > Toshi, Joerg, Michal!
> 
> Hi Thomas,
> 
> Thanks for checking. I was about to ping as well.
> 
> > I'm failing to find a conclusion of this discussion. Can we finally make
> > some progress with that?
> 
> I have not heard from Joerg since I last replied to his comments to
> Patch 3/3 -- I did my best to explain that there was no issue in the
> single page allocation in pud_free_pmd_page().  From my perspective, the
>  v3 series is good to go.

Well, I admit that this not my area but I agree with Joerg that
allocating memory inside afunction that is supposed to free page table
is far from ideal. More so that the allocation is hardcoded GFP_KERNEL.
We already have this antipattern in functions to allocate page tables
and it has turned to be maintenance PITA longterm. So if there is a way
around that then I would strongly suggest finding a different solution.

Whether that is sufficient to ditch the whole series is not my call
though.
-- 
Michal Hocko
SUSE Labs