linux-kernel - Re: [PATCH] x86/mm: Handle physical-virtual alignment mismatch in phys_p4d

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Message-ID: <20190624122355.762cadxds37enfdo@box>
Date:   Mon, 24 Jun 2019 15:23:55 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Baoquan He <bhe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kyle Pelton <kyle.d.pelton@...el.com>
Subject: Re: [PATCH] x86/mm: Handle physical-virtual alignment mismatch in
 phys_p4d_init()

On Mon, Jun 24, 2019 at 06:07:42PM +0800, Baoquan He wrote:
> On 06/21/19 at 01:54pm, Kirill A. Shutemov wrote:
> > > The code block as below is to zero p4d entries which are not coverred by
> > > the current memory range, and if haven't been mapped already. It's
> > > clearred away in this patch, could you also mention it in log, and tell
> > > why it doesn't matter now?
> > > 
> > > If it doesn't matter, should we clear away the simillar code in
> > > phys_pud_init/phys_pmd_init/phys_pte_init? Maybe a prep patch to do the
> > > clean up?
> > 
> > It only matters for the levels that contains page table entries that can
> > point to pages, not page tables. There's no p4d or pgd huge pages on x86.
> > Otherwise we only leak page tables without any benefit.
> 
> Ah, I checked git history, didn't find why it's added. I just Have a
> superficial knowledge of the clearing, but in a low-efficiency way.
> 
> > 
> > We might have this on all leveles under p?d_large() condition and don't
> > touch page tables at all.
> 
> I see.
> 
> > 
> > BTW, it all becomes rather risky for this late in the release cycle. Maybe
> > we should revert the original patch and try again later with more
> > comprehansive solution?
> 
> It's not added in one time. I am fine with your current change, would be
> much better if mention it in log, and also add code comment above the
> clearing code. Surely reverting and trying later with more comprehensive
> solution is also good to me, this need a little more effort.

I've decided to keep the block for now. We can remove it later, once the fixis in.
I'll post it soon

-- 
 Kirill A. Shutemov