Messages in this thread | | | Date | Tue, 15 Nov 2022 13:03:44 -0500 | From | Peter Xu <> | Subject | Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte |
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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.
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.
-- Peter Xu
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