Re: Would it be possible to add FGP_FIXED_ORDER for __filemap_get_folio()?

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 13, 2023 at 08:52:47AM +1030, Qu Wenruo wrote:
> I'm recently converting btrfs (for now only metadata) to go larger (order >
> 0) folios, and so far it looks pretty good (can already pass local fstests
> runs).
> 
> But to achieve that btrfs metadata is not utilizing __filemap_get_folio() at
> all, but manually allocating larger folios, then attaching them to the
> filemap.
> 
> This is due to 2 factors:
> 
> - We want fixed folio size
> - We have our own fallback path
> 
>   The whole idea is go large or go bust, either we got a large folio
>   matching our request exactly, or we fall back to the old per-page
>   allocation (the cross-page handling infrastructure is here anyway).

In the happy case, are all folios attached to the mapping of the same
order?  Or might you have one folio of eg, order-5 and another of
order-2 because they're different types of metadata?

I ask because we have patches out (not merged yet) that allow for
setting min/max order, and what you're asking for sounds like it could
be accommodated by that.