Re: [PATCH v2 01/18] mm: Make folios_put() the basis of release_pages()

[David Hildenbrand <david@redhat.com>]

On 17.02.24 03:25, Matthew Wilcox (Oracle) wrote:
> By making release_pages() call folios_put(), we can get rid of the calls
> to compound_head() for the callers that already know they have folios.
> We can also get rid of the lock_batch tracking as we know the size
> of the batch is limited by folio_batch.  This does reduce the maximum
> number of pages for which the lruvec lock is held, from SWAP_CLUSTER_MAX
> (32) to PAGEVEC_SIZE (15).  I do not expect this to make a significant
> difference, but if it does, we can increase PAGEVEC_SIZE to 31.
> 

I'm afraid that won't apply to current mm-unstable anymore, where we can 
now put multiple references to a single folio (as part of unmapping
large PTE-mapped folios).

[...]

> +/**
> + * release_pages - batched put_page()
> + * @arg: array of pages to release
> + * @nr: number of pages
> + *
> + * Decrement the reference count on all the pages in @arg.  If it
> + * fell to zero, remove the page from the LRU and free it.
> + *
> + * Note that the argument can be an array of pages, encoded pages,
> + * or folio pointers. We ignore any encoded bits, and turn any of
> + * them into just a folio that gets free'd.
> + */
> +void release_pages(release_pages_arg arg, int nr)
> +{
> +	struct folio_batch fbatch;
> +	struct encoded_page **encoded = arg.encoded_pages;
> +	int i;
> +
> +	folio_batch_init(&fbatch);
> +	for (i = 0; i < nr; i++) {
> +		/* Turn any of the argument types into a folio */
> +		struct folio *folio = page_folio(encoded_page_ptr(encoded[i]));
> +

As an "easy" way forward, we could handle these "multiple-ref" cases 
here by putting ref-1 references, and leaving the single remaining 
reference to folios_put().

That implies, more atomic operations, though.

Alternatively, "struct folio_batch" would have to be optimized to 
understand "put multiple references" as well.

> +		if (folio_batch_add(&fbatch, folio) > 0)
> +			continue;
> +		folios_put(&fbatch);
> +	}
> +
> +	if (fbatch.nr)
> +		folios_put(&fbatch);
>   }
>   EXPORT_SYMBOL(release_pages);
>   

-- 
Cheers,

David / dhildenb