linux-kernel - Re: [PATCH] arm64/mm: Avoid direct referencing page table enties in map

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Message-ID: <c65c0bcc-149e-4f30-9bab-e5377230d2cd@arm.com>
Date: Thu, 1 Aug 2024 13:48:17 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Will Deacon <will@...nel.org>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
 linux-arm-kernel@...ts.infradead.org,
 Catalin Marinas <catalin.marinas@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Avoid direct referencing page table enties in
 map_range()

On 01/08/2024 12:34, Will Deacon wrote:
> On Thu, Jul 25, 2024 at 11:36:56AM +0100, Ryan Roberts wrote:
>> On 25/07/2024 10:10, Anshuman Khandual wrote:
>>> Like else where in arm64 platform, use WRITE_ONCE() in map_range() while
>>> creating page table entries. This avoids referencing page table entries
>>> directly.
>>
>> I could be wrong, but I don't think this code is ever operating on live
>> pgtables? So there is never a potential to race with the HW walker and therefore
>> no need to guarrantee copy atomicity? As long as the correct barriers are placed
>> at the point where you load the pgdir into the TTBRx there should be no problem?
>>
>> If my assertion is correct, I don't think there is any need for this change.
> 
> Agreed.

I think I need to row back on this. It looks like map_range() does act on live
pgtables; see map_kernel() where twopass == true. init_pg_dir is populated then
installed in TTBR1, then permissions are modified with 3 [un]map_segment()
calls, which call through to map_range().

So on that basis, I think the WRITE_ONCE() calls are warranted. And to be
consistent, I'd additionally recommend adding a READ_ONCE() around the:

if (pte_none(*tbl)) {

Thanks,
Ryan

> 
> Will