Re: Issue with 8K folio size in __filemap_get_folio()

[David Hildenbrand <david@redhat.com>]

On 04.12.23 18:17, Matthew Wilcox wrote:
> On Mon, Dec 04, 2023 at 04:09:36PM +0100, David Hildenbrand wrote:
>> I think for the pagecache it should work. In the context of [1], a total
>> mapcount would likely still be possible. Anything beyond that likely not, if
>> we ever care.
> 
> I confess I hadn't gone through your patches.
> 
> https://lore.kernel.org/all/20231124132626.235350-8-david@redhat.com/
> 
> is the critical one.  It's actually going to walk off the end of order-2
> folios today (which we'll create, eg with XFS).

Note that order-2 only uses _rmap_val0. So that *should* work  as 
expected (and that layout resulted in the best cache behavior, weirdly 
enough).

> 
> You can put _rmap_val0 and _rmap_val1 in page2 and _rmap_val2-5 in page3
> to fix this.  Once we're allocating order-1 folios, I think you can
> avoid this scheme and just check page0 and page1 mapcounts independently.

I could put _rmap_val0 in page1 after I get rid of  		_folio_nr_pages. 
That would make it work on 64bit.

But yeah, it feels kind of stupid to do elaborate tracking for "it's 2 
pages". OTOH, less special-casing can be preferable.

-- 
Cheers,

David / dhildenb