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

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Message-ID: <E4287DB5-7D0D-41D7-BC0E-F5F404853931@nvidia.com>
Date: Fri, 07 Feb 2025 09:35:27 -0500
From: Zi Yan <ziy@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Ryan Roberts <ryan.roberts@....com>, Hugh Dickins <hughd@...gle.com>,
 David Hildenbrand <david@...hat.com>, Yang Shi <yang@...amperecomputing.com>,
 Miaohe Lin <linmiaohe@...wei.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 Yu Zhao <yuzhao@...gle.com>, John Hubbard <jhubbard@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/7] Buddy allocator like (or non-uniform) folio split

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