Re: [RFC PATCH 0/4] Extend xas_split* to support splitting arbitrarily large entries

[Ackerley Tng <ackerleytng@google.com>]

Matthew Wilcox <willy@infradead.org> writes:

> On Mon, Nov 17, 2025 at 02:46:57PM -0800, Ackerley Tng wrote:
>> guest_memfd is planning to store huge pages in the filemap, and
>> guest_memfd's use of huge pages involves splitting of huge pages into
>> individual pages. Splitting of huge pages also involves splitting of
>> the filemap entries for the pages being split.

>
> Hm, I'm not most concerned about the number of nodes you're allocating.

Thanks for reminding me, I left this out of the original message.

Splitting the xarray entry for a 1G folio (in a shift-18 node for
order=18 on x86), assuming XA_CHUNK_SHIFT is 6, would involve

+ shift-18 node (the original node will be reused - no new allocations)
+ shift-12 node: 1 node allocated
+ shift-6 node : 64 nodes allocated
+ shift-0 node : 64 * 64 = 4096 nodes allocated

This brings the total number of allocated nodes to 4161 nodes. struct
xa_node is 576 bytes, so that's 2396736 bytes or 2.28 MB, so splitting a
1G folio to 4K pages costs ~2.5 MB just in filemap (XArray) entry
splitting. The other large memory cost would be from undoing HVO for the
HugeTLB folio.

> I'm most concerned that, once we have memdescs, splitting a 1GB page
> into 512 * 512 4kB pages is going to involve allocating about 20MB
> of memory (80 bytes * 512 * 512).

I definitely need to catch up on memdescs. What's the best place for me
to learn/get an overview of how memdescs will describe memory/replace
struct folios?

I think there might be a better way to solve the original problem of
usage tracking with memdesc support, but this was intended to make
progress before memdescs.

> Is this necessary to do all at once?

The plan for guest_memfd was to first split from 1G to 4K, then optimize
on that by splitting in stages, from 1G to 2M as much as possible, then
to 4K only for the page ranges that the guest shared with the host.