Re: [PATCH v1] mm: centralize+fix comments about compound_mapcount() in new sync_with_folio_pmd_zap()

[David Hildenbrand (Arm)]

On 2/23/26 18:58, Matthew Wilcox wrote:
> On Mon, Feb 23, 2026 at 05:39:20PM +0100, David Hildenbrand (Arm) wrote:
>>  void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);
>>  
>> +/**
>> + * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD
>> + * @mm: The mm_struct.
>> + * @pmdp: Pointer to the pmd that was found to be pmd_none().
>> + *
>> + * When we stumble over a pmd_none() without holding the PTL while unmapping a
>> + * folio that could have been mapped at that PMD, it could be that concurrent
>> + * zapping of the PMD is not complete yet. While the PMD might be pmd_none()
>> + * already, the folio might still appear to be mapped (folio_mapped()).
>> + *
>> + * Wait for concurrent zapping to complete by grabbing the PTL.
>> + */
> 
> I like this.  The one thing we've lost is the name of the function which
> does the zapping, which I think was a helpful detail.  

I dropped it as we seem to have a talent to rename functions but to
forget about updating comments.

This very patch is an example of that :)

But well, I used folio_mapped() ... so ...

> Perhaps not to
> someone who's deep in "how page tables work", but I wouldn't know where
> to look for the counterpart to this.  So how about:
> 
> Option A:
> + * When we stumble over a pmd_none() without holding the PTL while
> + * unmapping a folio that could have been mapped at that PMD, 
> + * zap_huge_pmd() may not be complete yet. While the PMD might be pmd_none()
> + * already, the folio might still appear to be mapped (folio_mapped()).
> 
> Option B:
> + * When we find a pmd_none() while unmapping a folio without holding
> + * the PTL, zap_huge_pmd() may have cleared the PMD but not yet
> + * modified the folio to indicate that it's unmapped.
Let's do a slightly longer B:

* When we find a pmd_none() while unmapping a folio without holding
* the PTL, zap_huge_pmd() may have cleared the PMD but not yet
* modified the folio to indicate that it's unmapped. Skipping the PMD
* without synchronization could make folio unmapping code assume
* that unmapping failed.

Good? If so, Andrew can you squash or should I resend?

Thanks for the review!

-- 
Cheers,

David