Re: [PATCH 2/9] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

[Dev Jain <dev.jain@arm.com>]

On 10/03/26 2:09 pm, Lorenzo Stoakes (Oracle) wrote:
> On Tue, Mar 10, 2026 at 02:01:44PM +0530, Dev Jain wrote:
>>
>>
>> On 10/03/26 1:40 pm, Lorenzo Stoakes (Oracle) wrote:
>>> On Tue, Mar 10, 2026 at 01:00:06PM +0530, Dev Jain wrote:
>>>> Initialize nr_pages to 1 at the start of the loop, similar to what is being
>>>> done in folio_referenced_one(). It may happen that the nr_pages computed
>>>> from a previous call to folio_unmap_pte_batch gets reused without again
>>>> going through folio_unmap_pte_batch, messing up things. Although, I don't
>>>> think there is any bug right now; a bug would have been there, if in the
>>>> same instance of a call to try_to_unmap_one, we end up in the
>>>> pte_present(pteval) branch, then in the else branch doing pte_clear() for
>>>> device-exclusive ptes. This means that a lazyfree folio has some present
>>>> entries and some device entries mapping it. Since a pte being
>>>> device-exclusive means that a GUP reference on the underlying folio is
>>>> held, the lazyfree unmapping path upon witnessing this will abort
>>>> try_to_unmap_one.
>>>
>>> Dude, paragraphs. PARAGRAPHS :) this is one dense set of words.
>>>
>>> It's also a very compressed 'stream of consciousness' hard-to-read block here.
>>
>> Sure :) I'll try to break this down.
>>
>>>
>>> I'm not sure it's really worth having this as a separate commit either, it's
>>> pretty trivial.
>>
>> Hmm...well, as I explain above, it's not trivial for me :) it is difficult
>> for me to reason here whether nr_pages can be reused without a reset in
>> a future iteration.
> 
> OK you can have it be separate, let's just really clean up the commit message
> then please :)

Okay, thanks.

> 
> Thanks, Lorenzo