Re: [PATCH v1 1/2] mm/huge_memory: introduce uniform_split_unmapped_folio_to_zero_order

[Harry Yoo <harry.yoo@oracle.com>]

On Sun, Nov 16, 2025 at 10:21:26PM -0500, Zi Yan wrote:
> On 16 Nov 2025, at 22:15, Harry Yoo wrote:
> 
> > 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.
> 
> Why not use __split_unmaped_folio(folio, /*new_order=*/0,
> 								  /split_at=*/HWPoisoned_page,
> 								  ..., /*uniform_split=*/ false)?
> 
> If there are multiple HWPoisoned pages, just repeat.

Using __split_unmapped_folio() is totally fine. I was just thinking that
maybe we should hide the complexity inside the page allocator if we want
to avoid allocating struct folio at all when handling this.

> > 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