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 13:03:44 -0500
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...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 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