Re: Improve page refcount scalability

[David Hildenbrand (Arm)]

On 10/16/26 16:40, Kiryl Shutsemau wrote:
> On Tue, Oct 13, 2026 at 11:43:14AM -0700, Linus Torvalds wrote:
>> Comments?
> 
> There's a regression: the lockless read loses on large reads from large
> folios.
> 
> In a 64-vCPU guest on a Threadripper 7975WX, 1 MiB reads of a cached
> ext4 file are 9% slower from one thread with rcu-free-v3 than without
> it, 64K reads 4% slower.
> 
> The reference path looks a large folio up once and copies it in one go.
> The lockless one walks i_pages from the root and calls
> copy_page_to_iter() for every 4K page.
> 
> I've added a cutoff to the lockless path: if the folio is large and the
> read covers more than four of its pages, leave it to the slow path,
> where one reference covers all of them.
> 
> 		if (folio != prev) {
> 			if (!filemap_read_rcu_can_copy(folio, index, *last_index))
> 				break;
> 			/* Many pages of a large folio: one reference is cheaper */
> 			if (folio_test_large(folio) &&
> 			    min_t(unsigned long, folio_next_index(folio) - index,
> 				  DIV_ROUND_UP(offset + iov_iter_count(iter),
> 					       PAGE_SIZE)) > 4)
> 				break;
> 			prev = folio;
> 		}
> 
> With it, ext4 1 MiB reads match base and 64K reads are within 2%. Small
> reads do not change. The cut is on the folio, not on the read size, so
> files in 4K folios keep the gain on large reads: 1 MiB tmpfs reads from
> one thread are ~30% faster than base.
> 
> Sounds reasonable?
> 
> I've pushed my tree here:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git rcu-free/rfc-v1
> 
> I'm thinking of posting it as an RFC to make it easier to review.
> Objections?

Yes please. I spent 6 hours today fixing all sorts of bugs after wanting to get
something rather trivial fixed.

I need the patch in my inbox to not get distracted by all the other stuff :)

-- 
Cheers,

David