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

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Message-ID: <aRuuRGxw2vuXcVv6@casper.infradead.org>
Date: Mon, 17 Nov 2025 23:22:44 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, david@...hat.com,
	michael.roth@....com, vannapurve@...gle.com
Subject: Re: [RFC PATCH 0/4] Extend xas_split* to support splitting
 arbitrarily large entries

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.
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).  Is this necessary to do all at once?