linux-kernel - Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte

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Date:   Tue, 15 Nov 2022 19:08:01 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ives van Hoorne <ives@...esandbox.io>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when
 recover pte

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