linux-kernel - Re: [PATCH 2/3] mm/huge_memory.c: update tlb entry if pmd is changed

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Date:   Thu, 25 Jun 2020 17:57:02 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Daniel Silsby <dansilsby@...il.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/3] mm/huge_memory.c: update tlb entry if pmd is changed



On 06/25/2020 08:30 AM, Mike Kravetz wrote:
> On 6/24/20 2:26 AM, Bibo Mao wrote:
>> When set_pmd_at is called in function do_huge_pmd_anonymous_page,
>> new tlb entry can be added by software on MIPS platform.
>>
>> Here add update_mmu_cache_pmd when pmd entry is set, and
>> update_mmu_cache_pmd is defined as empty excepts arc/mips platform.
>> This patch has no negative effect on other platforms except arc/mips
>> system.
> 
> I am confused by this comment.  It appears that update_mmu_cache_pmd
> is defined as non-empty on arc, mips, powerpc and sparc architectures.
> Am I missing something?
ohh, sparc is missing here, it not defined as empty. On powerpc it is defined
as empty.

> 
> If those architectures do provide update_mmu_cache_pmd, then the previous
> patch and this one now call update_mmu_cache_pmd with the actual faulting
> address instead of the huge page aligned address.  This was intentional
> for mips.  However, are there any potential issues on the other architectures?
It is not special for mips, only that fault address is useful on mips system.
In function huge_pmd_set_accessed/do_huge_pmd_wp_page, update_mmu_cache_pmd is
called with vmf->address, rather than start address of pmd page.

regards
bibo,mao

> I am no expert in any of those architectures.  arc looks like it could be
> problematic as update_mmu_cache_pmd calls update_mmu_cache and then
> operates on (address & PAGE_MASK).  That could now be different.