linux-kernel - Re: [PATCH v1 4/9] fs/dax: use vmf_insert_folio

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Message-ID: <705fa84f-7a4a-4f74-82bf-5dff51149d94@redhat.com>
Date: Thu, 17 Jul 2025 10:39:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "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>,
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 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 v1 4/9] fs/dax: use vmf_insert_folio_pmd() to insert the
 huge zero folio

On 17.07.25 10:38, Alistair Popple wrote:
> On Tue, Jul 15, 2025 at 03:23:45PM +0200, David Hildenbrand wrote:
>> Let's convert to vmf_insert_folio_pmd().
>>
>> There is a theoretical change in behavior: in the unlikely case there is
>> already something mapped, we'll now still call trace_dax_pmd_load_hole()
>> and return VM_FAULT_NOPAGE.
>>
>> Previously, we would have returned VM_FAULT_FALLBACK, and the caller
>> would have zapped the PMD to try a PTE fault.
>>
>> However, that behavior was different to other PTE+PMD faults, when there
>> would already be something mapped, and it's not even clear if it could
>> be triggered.
>>
>> Assuming the huge zero folio is already mapped, all good, no need to
>> fallback to PTEs.
>>
>> Assuming there is already a leaf page table ... the behavior would be
>> just like when trying to insert a PMD mapping a folio through
>> dax_fault_iter()->vmf_insert_folio_pmd().
>>
>> Assuming there is already something else mapped as PMD? It sounds like
>> a BUG, and the behavior would be just like when trying to insert a PMD
>> mapping a folio through dax_fault_iter()->vmf_insert_folio_pmd().
>>
>> So, it sounds reasonable to not handle huge zero folios differently
>> to inserting PMDs mapping folios when there already is something mapped.
> 
> Yeah, this all sounds reasonable and I was never able to hit this path with the
> RFC version of this series anyway. So I suspect it really is impossible to hit
> and therefore any change is theoretical.

Thanks for the review and test, Alistair!

-- 
Cheers,

David / dhildenb