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

[Zi Yan]

On 7 Feb 2025, at 9:25, Matthew Wilcox wrote:

> On Fri, Feb 07, 2025 at 09:11:39AM -0500, Zi Yan wrote:
>> Existing uniform split requires 2^(order % XA_CHUNK_SHIFT) xa_node allocations
>> during split, when the folio needs to be split to order-0. But non-uniform split
>> only requires at most 1 xa_node allocation. For example, to split an order-9
>> folio, 8 xa_nodes are needed for uniform split, since the folio takes 8
>> multi-index slots in the xarray. But for non-uniform split, only the slot
>> containing the given struct page needs a xa_node after the split. There will be
>> a 7 xa_node saving.
>>
>> Hi Matthew,
>>
>> Do you mind checking my statement above on xarray memory saving? And correct me
>> if I miss anything. Thanks.
>
> We currently have a bug where we can't split order-12 (or above) to order-0 (or anything in the range 0-5) as we'd need to allocate two layers of nodes, and
> the preallocation can't do that.
>
> 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).


Best Regards,
Yan, Zi