Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <dc5a5173-deeb-a6d0-6c2f-5f6f448bf83e@redhat.com>
Date:   Tue, 15 Nov 2022 19:08:01 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.1
Subject: Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when
 recover pte
Content-Language: en-US
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrea Arcangeli <aarcange@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Ives van Hoorne <ives@codesandbox.io>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>, stable@vger.kernel.org
References: <20221110203132.1498183-1-peterx@redhat.com>
 <20221110203132.1498183-2-peterx@redhat.com>
 <9af36be3-313b-e39c-85bb-bf30011bccb8@redhat.com> <Y3KgYeMTdTM0FN5W@x1n>
 <ec8b3c86-d3b2-f898-7297-c20a58ae2ac1@redhat.com> <Y3O5bCXSbvKJrjRL@x1n>
 <82d7a142-8c78-4168-37e9-7b677b18987a@redhat.com> <Y3PUgOUYx6ECN405@x1n>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <Y3PUgOUYx6ECN405@x1n>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 15.11.22 19:03, Peter Xu wrote:
> On Tue, Nov 15, 2022 at 06:22:03PM +0100, David Hildenbrand wrote:
>> That's precisely what I had in mind recently, and I am happy to hear that
>> you have similar idea:
>>
>> https://lkml.kernel.org/r/20221108174652.198904-6-david@redhat.com
>>
>> "
>> Note that we don't optimize for the actual migration case:
>> (1) When migration succeeds the new PTE will not be writable because the
>>      source PTE was not writable (protnone); in the future we
>>      might just optimize that case similarly by reusing
>>      can_change_pte_writable()/can_change_pmd_writable() when removing
>>      migration PTEs.
>> "
> 
> I see, sorry I haven't yet read it, but sounds doable indeed.
> 
>>
>> Currently, "readable_migration_entry" is even wrong: it might be PROT_NONE
>> and not even readable.
> 
> Do you mean mprotect(PROT_NONE)?
> 
> If we read the "read migration entry" as "migration entry with no write
> bit", it seems still fine, and code-wise after pte recovered it should
> still be PROT_NONE iiuc because mk_pte() will just make a pte without
> e.g. _PRESENT bit set on x86 while it'll have the _PROT_NONE bit.

Exactly that's the unintuitive interpretation of 
"readable_migration_entry". By "wrong" I meant: the naming is wrong.

> 
> May not keep true for numa balancing though: when migration happens after a
> numa hint applied to a pte, it seems to me it's prone to lose the hint
> after migration completes (assuming this migration is not the numa
> balancing operation itself caused by a page access).  Doesn't sound like a
> severe issue though even if I didn't miss something, since if the page got
> moved around the original hint may need to reconsider anyway.

Yes, I think any migration will lose fake PROT_NONE. "Fake" as in "not 
VMA permissions" but "additional permissions imposed by NUMA hinting 
faults."

-- 
Thanks,

David / dhildenb