Re: [PATCH] mm/mremap: Honour writable bit in mremap pte batching

[David Hildenbrand <david@redhat.com>]

On 28.10.25 12:48, Pedro Falcato wrote:
> On Tue, Oct 28, 2025 at 12:09:52PM +0530, Dev Jain wrote:
>> Currently mremap folio pte batch ignores the writable bit during figuring
>> out a set of similar ptes mapping the same folio. Suppose that the first
>> pte of the batch is writable while the others are not - set_ptes will
>> end up setting the writable bit on the other ptes, which is a violation
>> of mremap semantics. Therefore, use FPB_RESPECT_WRITE to check the writable
>> bit while determining the pte batch.
>>
> 
> Hmm, it seems to be like we're doing the wrong thing by default here?
> I must admit I haven't followed the contpte work as much as I would've
> liked, but it doesn't make much sense to me why FPB_RESPECT_WRITE would
> be an option you have to explicitly pass, and where folio_pte_batch (the
> "simple" interface) doesn't Just Do The Right Thing for naive callers.

We use the "simple" version to apply to as many callers as possible: the 
common case, not some "let's be super careful" scenarios.

> 
> Auditing all callers:
>   - khugepaged clears a variable number of ptes
>   - memory.c clears a variable number of ptes
>   - mempolicy.c grabs folios for migrations
>   - mlock.c steps over nr_ptes - 1 ptes, speeding up traversal
>   - mremap is borked since we're remapping nr_ptes ptes
>   - rmap.c TTU unmaps nr_ptes ptes for a given folio
> 
>   so while the vast majority of callers don't seem to care, it would make
>   sense that folio_pte_batch() works conservatively by default, and
>   folio_pte_batch_flags() would allow for further batching (or maybe
>   we would add a separate folio_pte_batch_clear() or
>   folio_pte_batch_greedy() or whatnot).

I think really the tricky part is when we'e not only scanning or 
clearing, but actually want to "set" ptes again based on the result, 
like we do here.

For that we could consider having a second variant. But if it ends up 
having only a single caller, it's also not that great.

> 
>> Cc: stable@vger.kernel.org #6.17
>> Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching")
>> Reported-by: David Hildenbrand <david@redhat.com>
>> Debugged-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
> 
> But the solution itself looks okay to me. so, fwiw:
> 
> Acked-by: Pedro Falcato <pfalcato@suse.de>
> 

Backport might end up being a bit tricky I suspect.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb