Re: [PATCH v6 0/7] Buddy allocator like (or non-uniform) folio split

[Matthew Wilcox]

On Fri, Feb 07, 2025 at 09:35:27AM -0500, Zi Yan wrote:
> On 7 Feb 2025, at 9:25, Matthew Wilcox wrote:
> > As part of your series, I'd like to remove that limitation, so we'd need
> > to allocate log_64(n - m) [ok, more complex than that, but ykwim].  So
> > it's not quite "only allocate one node", but it's allocate O(log(current
> > number of nodes needed to be allocated)).
> >
> > Makes sense?
> 
> Yes.
> 
> To remove that order-12 limitation, do shmem_split_large_entry() and
> __filemap_add_folio() need some change as well? Both call xas_split_alloc().
> But I do not know if they will see splitting order-12 to order-(0 to 5).

__filemap_add_folio() doesn't need to fracture like it currently does;
it can do the same minimum split.  The situation is that we've got a
shadow entry which covers 2^n slots, and now we want to add a folio
which only covers 2^m slots with m < n.  Leaving n-m shadow entries in the tree
with orders ranging from m to n-1 makes more sense than the eager split.

shmem is the same, except that it's storing swap entries instead of
shadow entries.