Messages in this thread | | | Date | Tue, 15 Nov 2022 19:08:01 +0100 | Subject | Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte | From | David Hildenbrand <> |
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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
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