Re: [PATCH v2 5/5] mm/mmu_gather: Store and process pages in contig ranges

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

On 30/08/2023 16:07, Matthew Wilcox wrote:
> On Wed, Aug 30, 2023 at 10:50:11AM +0100, Ryan Roberts wrote:
>> +++ b/include/asm-generic/tlb.h
>> @@ -246,11 +246,11 @@ struct mmu_gather_batch {
>>  	struct mmu_gather_batch	*next;
>>  	unsigned int		nr;
>>  	unsigned int		max;
>> -	struct page		*pages[];
>> +	struct pfn_range	folios[];
> 
> I think it's dangerous to call this 'folios' as it lets you think that
> each entry is a single folio.  But as I understand this patch, you can
> coagulate contiguous ranges across multiple folios.

No that's not quite the case; each contiguous range only ever spans a *single*
folio. If there are 2 contiguous folios, they will be represented as separate
ranges. This is done so that we can subsequently do the per-folio operations
without having to figure out how many folios are within each range - one range =
one (contiguous part of a) folio.

On naming, I was calling this variable "ranges" in v1 but thought folios was
actually clearer. How about "folio_regions"?

> 
>> -void free_pages_and_swap_cache(struct page **pages, int nr)
>> +void free_folios_and_swap_cache(struct pfn_range *folios, int nr)
>>  {
>>  	lru_add_drain();
>>  	for (int i = 0; i < nr; i++)
>> -		free_swap_cache(pages[i]);
>> -	release_pages(pages, nr);
>> +		free_swap_cache(pfn_to_page(folios[i].start));
> 
> ... but here, you only put the swapcache for the first folio covered by
> the range, not for each folio.

Yes that's intentional - one range only ever covers one folio, so I only need to
call free_swap_cache() once for the folio. Unless I've misunderstood and
free_swap_cache() is actually decrementing a reference count and needs to be
called for every page? (but it doesn't look like that in the code).

> 
>> +	folios_put_refs(folios, nr);
> 
> It's kind of confusing to have folios_put() which takes a struct folio *
> and then folios_put_refs() which takes a struct pfn_range *.
> pfn_range_put()?

I think it's less confusing if you know that each pfn_range represents a single
contig range of pages within a *single* folio. pfn_range_put() would make it
sound like its ok to pass a pfn_range that spans multiple folios (this would
break). I could rename `struct pfn_range` to `struct sub_folio` or something
like that. Would that help make the semantic clearer?