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

David Hildenbrand
2022-11-15T18:08:09Z

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