linux-kernel - Re: [PATCH v1 1/2] mm/huge_memory: introduce uniform_split_unmapped_folio_to_zero

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Message-ID: <aRqTLmJBuvBcLYMx@hyeyoo>
Date: Mon, 17 Nov 2025 12:15:23 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Jiaqi Yan <jiaqiyan@...gle.com>, nao.horiguchi@...il.com,
        linmiaohe@...wei.com, ziy@...dia.com, david@...hat.com,
        lorenzo.stoakes@...cle.com, william.roche@...cle.com,
        tony.luck@...el.com, wangkefeng.wang@...wei.com, jane.chu@...cle.com,
        akpm@...ux-foundation.org, osalvador@...e.de, muchun.song@...ux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v1 1/2] mm/huge_memory: introduce
 uniform_split_unmapped_folio_to_zero_order

On Sun, Nov 16, 2025 at 11:51:14AM +0000, Matthew Wilcox wrote:
> On Sun, Nov 16, 2025 at 01:47:20AM +0000, Jiaqi Yan wrote:
> > Introduce uniform_split_unmapped_folio_to_zero_order, a wrapper
> > to the existing __split_unmapped_folio. Caller can use it to
> > uniformly split an unmapped high-order folio into 0-order folios.
> 
> Please don't make this function exist.  I appreciate what you're trying
> to do, but let's try to do it differently?
> 
> When we have struct folio separately allocated from struct page,
> splitting a folio will mean allocating new struct folios for every
> new folio created.  I anticipate an order-0 folio will be about 80 or
> 96 bytes.  So if we create 512 * 512 folios in a single go, that'll be
> an allocation of 20MB.
>
> This is why I asked Zi Yan to create the asymmetrical folio split, so we
> only end up creating log() of this.  In the case of a single hwpoison page
> in an order-18 hugetlb, that'd be 19 allocations totallying 1520 bytes.

Oh god, I completely overlooked this aspect when discussing this with Jiaqi.
Thanks for raising this concern.

> But since we're only doing this on free, we won't need to do folio
> allocations at all; we'll just be able to release the good pages to the
> page allocator and sequester the hwpoison pages.

[+Cc PAGE ALLOCATOR folks]

So we need an interface to free only healthy portion of a hwpoison folio.

I think a proper approach to this should be to "free a hwpoison folio
just like freeing a normal folio via folio_put() or free_frozen_pages(),
then the page allocator will add only healthy pages to the freelist and
isolate the hwpoison pages". Oherwise we'll end up open coding a lot,
which is too fragile.

In fact, that can be done by teaching free_pages_prepare() how to handle
the case where one or more subpages of a folio are hwpoison pages.

How this should be implemented in the page allocator in memdescs world?
Hmm, we'll want to do some kind of non-uniform split, without actually
splitting the folio but allocating struct buddy?

But... for now I think hiding this complexity inside the page allocator
is good enough. For now this would just mean splitting a frozen page
inside the page allocator (probably non-uniform?). We can later re-implement
this to provide better support for memdescs.

-- 
Cheers,
Harry / Hyeonggon