linux-kernel - Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

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Date:   Mon, 23 Aug 2021 20:08:34 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "rppt@...nel.org" <rppt@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "Lutomirski, Andy" <luto@...nel.org>
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

On Mon, 2021-08-23 at 16:25 +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> Allocate page table using __GFP_PTE_MAPPED so that they will have 4K
> PTEs
> in the direct map. This allows to switch _PAGE_RW bit each time a
> page
> table page needs to be made writable or read-only.
> 
> The writability of the page tables is toggled only in the lowest
> level page
> table modifiction functions and immediately switched off.
> 
> The page tables created early in the boot (including the direct map
> page
> table) are not write protected.
> 
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
I have a second version of the PKS tables series that I think gets all
of them.

Also, I didn't see any flush anywhere when toggling. I guess the
spurious kernel fault handler is doing the work? It might be better to
just do a local flush of the address.