linux-kernel - Re: [PATCH v2 8/9] mm: introduce and use vm_normal_page

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Message-ID: <c578bdc9-38fc-4811-b69a-a734a82f073f@lucifer.local>
Date: Fri, 18 Jul 2025 13:44:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, linux-fsdevel@...r.kernel.org,
        nvdimm@...ts.linux.dev, Andrew Morton <akpm@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Hugh Dickins <hughd@...gle.com>, Oscar Salvador <osalvador@...e.de>,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v2 8/9] mm: introduce and use vm_normal_page_pud()

On Fri, Jul 18, 2025 at 01:06:30PM +0200, David Hildenbrand wrote:
> On 18.07.25 12:47, Lorenzo Stoakes wrote:
> > On Thu, Jul 17, 2025 at 10:14:33PM +0200, David Hildenbrand wrote:
> > > On 17.07.25 22:03, Lorenzo Stoakes wrote:
> > > > On Thu, Jul 17, 2025 at 01:52:11PM +0200, David Hildenbrand wrote:
> > > > > Let's introduce vm_normal_page_pud(), which ends up being fairly simple
> > > > > because of our new common helpers and there not being a PUD-sized zero
> > > > > folio.
> > > > >
> > > > > Use vm_normal_page_pud() in folio_walk_start() to resolve a TODO,
> > > > > structuring the code like the other (pmd/pte) cases. Defer
> > > > > introducing vm_normal_folio_pud() until really used.
> > > >
> > > > I mean fine :P but does anybody really use this?
> > >
> > > This is a unified PFN walker (!hugetlb + hugetlb), so you can easily run
> > > into hugetlb PUDs, DAX PUDs and huge pfnmap (vfio) PUDs :)
> >
> > Ahhh ok. I hate hugetlb so very very much.
> >
> > Oscar is doing the Lord's work improving things but the trauma is still
> > there... :P
> >
> > Also yeah DAX ahem.
> >
> > I'm not familiar with huge pfnmap PUDs, could you give me a hint on this? :>)
>
> vmf_insert_pfn_pmd(), called from  drivers/vfio/pci/vfio_pci_core.c
>
> Essentially, we create huge PUDs when mapping device BARs to user space.

Ah makes sense. Thanks!