linux-kernel - Re: [PATCH] xen: fix usage of pmd/pud

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Date:   Wed, 8 Sep 2021 13:07:59 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org,
        Sander Eikelenboom <linux@...elenboom.it>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen: fix usage of pmd/pud_poplulate in mremap for pv
 guests

On 08.09.2021 09:36, Juergen Gross wrote:
> Commit 0881ace292b662 ("mm/mremap: use pmd/pud_poplulate to update page
> table entries") introduced a regression when running as Xen PV guest.

The description of that change starts with "pmd/pud_populate is the
right interface to be used to set the respective page table entries."
If this is deemed true, I don't think pmd_populate() should call
paravirt_alloc_pte(): The latter function, as its name says, is
supposed to be called for newly allocated page tables only (aiui).

> Today pmd/pud_poplulate() for Xen PV assumes that the PFN inserted is
> referencing a not yet used page table. In case of move_normal_pmd/pud()
> this is not true, resulting in WARN splats like:

I agree for the PMD part, but is including PUD here really correct?
While I don't know why that is, xen_alloc_ptpage() pins L1 tables
only. Hence a PUD update shouldn't be able to find a pinned L2
table.

Jan