linux-kernel - Re: [PATCH v3 3/3] mm/memory.c: Add memory read privilege before filling PTE entry

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Date:   Wed, 27 May 2020 13:55:18 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     maobibo <maobibo@...ngson.cn>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhc@...ote.com>,
        Paul Burton <paulburton@...nel.org>,
        Dmitry Korotin <dkorotin@...ecomp.com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>,
        Stafford Horne <shorne@...il.com>,
        Steven Price <steven.price@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "Maciej W. Rozycki" <macro@....com>, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v3 3/3] mm/memory.c: Add memory read privilege before
 filling PTE entry

On Tue, 19 May 2020, maobibo wrote:
> On 05/19/2020 04:57 AM, Andrew Morton wrote:
> > On Mon, 18 May 2020 13:08:49 +0800 Bibo Mao <maobibo@...ngson.cn> wrote:
> > 
> >> On mips platform, hw PTE entry valid bit is set in pte_mkyoung
> >> function, it is used to set physical page with readable privilege.
> > 
> > pte_mkyoung() seems to be a strange place to set the pte's valid bit. 
> > Why is it done there?  Can it be done within mips's mk_pte()?
> On MIPS system hardware cannot set PAGE_ACCESS bit when accessing the page,
> software sets PAGE_ACCESS software bit and PAGE_VALID hw bit together during page
> fault stage.
> 
> If mk_pte is called in page fault flow, it is ok to set both bits. If it is not 
> called in page fault, PAGE_ACCESS is set however there is no actual memory accessing.

Sorry for joining in so late, but would you please explain that some more:
preferably in the final commit message, if not here.

I still don't understand why this is not done in the same way as on other
architectures - those that care (I just checked x86, powerpc, arm, arm64,
but not all of them) make sure that all the bits they want are there in
mm/mmap.c's protection_map[16], which then feeds into vma->vm_page_prot,
and so into mk_pte() as Andrew indicated.

And I can see that arch/mips/mm/cache.c has a setup_protection_map()
to do that: why does it not set the additional bits that you want?
including the valid bit and the accessed (young) bit, as others do.
Are you saying that there are circumstances in which it is wrong
for mk_pte() to set the additional bits?

I'm afraid that generic mm developers will have no clue as to whether
or not to add a pte_sw_mkyoung() after a mk_pte(); and generic source
will be the cleaner if it turns out not to be needed (but thank you
for making sure that it does nothing on the other architectures).

Hugh