Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

[Ryan Roberts <ryan.roberts@arm.com>]

On 07/08/2025 20:20, Lorenzo Stoakes wrote:
> +cc Ryan for ContPTE stuff.

Appologies, I was aware of the other thread and on-going issues but haven't had
the bandwidth to follow too closely.

> 
> On Thu, Aug 07, 2025 at 09:10:52PM +0200, David Hildenbrand wrote:
>> Acked-by: David Hildenbrand <david@redhat.com>
> 
> Thanks!
> 
>>
>> Wondering whether we could then just use the patch hint instead of going via
>> the folio.
>>
>> IOW,
>>
>> return pte_batch_hint(ptep, pte);
> 
> Wouldn't that break the A/D stuff? Also this doesn't mean that the PTE won't
> have some conflicting flags potentially. The check is empirical:
> 
> static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
> {
> 	if (!pte_valid_cont(pte))
> 		return 1;
> 
> 	return CONT_PTES - (((unsigned long)ptep >> 3) & (CONT_PTES - 1));
> }
> 
> So it's 'the most number of PTEs that _might_ coalesce'.

No that's not correct; It's "at least this number of ptes _do_ coalesce".
folio_pte_batch() may end up returning a larger batch, but never smaller.

This function is looking to see if ptep is inside a conpte mapping, and if it
is, it's returning the number of ptes to the end of the contpte mapping (which
is of 64K size and alignment on 4K kernels). A contpte mapping will only exist
if the physical memory is appropriately aligned/sized and all belongs to a
single folio.

> 
> (note that a bit grossly we'll call it _again_ in folio_pte_batch_flags()).
> 
> I suppose we could not even bother with checking if same folio and _just_ check
> if PTEs have consecutive PFNs, which is not very likely if different folio
> but... could that break something?

Yes something could break; the batch must *all* belong to the same folio.
Functions like set_ptes() require that in their documentation, and arm64 depends
upon it in order not to screw up the access/dirty bits.

> 
> It seems the 'magic' is in set_ptes() on arm64 where it'll know to do the 'right
> thing' for a contPTE batch (I may be missing something - please correct me if so
> Dev/Ryan).

It will all do the right thing functionally no matter how you call it. But if
you can set_ptes() (and friends) on full contpte mappings, things are more
efficient.

> 
> So actually do we even really care that much about folio?

From arm64's perspective, we're happy enough with batches the size of
pte_batch_hint(). folio_pte_batch() is a bonus, but certainly not a deal-breaker
for this location.

For the record, I'm pretty sure I was the person pushing for protecting
vm_normal_folio() with pte_batch_hint() right at the start of this process :)

Thanks,
Ryan

> 
>>
>>
>> Not sure if that was discussed at some point before we went into the
>> direction of using folios. But there really doesn't seem to be anything
>> gained for other architectures here (as raised by Jann).
> 
> Yup... I wonder about the other instances of this... ruh roh.

IIRC prior to Dev's mprotect and mremap optimizations, I believe all sites
already needed the folio. I haven't actually looked at how mprotect ended up,
but maybe worth checking to see if it should protect with pte_batch_hint() too.

Thanks,
Ryan

> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
>>
> 
> Cheers, Lorenzo