linux-kernel - Re: [PATCH RFC 07/14] fs/dax: use vmf_insert_folio

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Message-ID: <36a8f286-1b09-43bd-9efa-5831ef3f315b@redhat.com>
Date: Fri, 4 Jul 2025 15:22:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.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>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH RFC 07/14] fs/dax: use vmf_insert_folio_pmd() to insert
 the huge zero folio

On 25.06.25 11:03, David Hildenbrand wrote:
> On 24.06.25 03:16, Alistair Popple wrote:
>> On Tue, Jun 17, 2025 at 05:43:38PM +0200, David Hildenbrand wrote:
>>> Let's convert to vmf_insert_folio_pmd().
>>>
>>> In the unlikely case there is already something mapped, we'll now still
>>> call trace_dax_pmd_load_hole() and return VM_FAULT_NOPAGE.
>>>
>>> That should probably be fine, no need to add special cases for that.
>>
>> I'm not sure about that. Consider dax_iomap_pmd_fault() -> dax_fault_iter() ->
>> dax_pmd_load_hole(). It calls split_huge_pmd() in response to VM_FAULT_FALLBACK
>> which will no longer happen, what makes that ok?
> 
> My reasoning was that this is the exact same behavior other
> vmf_insert_folio_pmd() users here would result in.
> 
> But let me dig into the details.

Okay, trying to figure out what to do here.

Assume dax_pmd_load_hole() is called and there is already something. We 
would have returned VM_FAULT_FALLBACK, now we would return VM_FAULT_NO_PAGE.

That obviously only happens when we have not a write fault (otherwise, 
the shared zeropage does not apply).

In dax_iomap_pmd_fault(), we would indeed split_huge_pmd(). In the DAX 
case (!anon vma), that would simply zap whatever is already mapped there.

I guess we would then return VM_FAULT_FALLBACK from huge_fault-> ... -> 
dax_iomap_fault() and core MM code would fallback to handle_pte_fault() 
etc. and ... load a single PTE mapping the shared zeropage.

BUT

why is this case handled differently than everything else?

E.g.,

(1) when we try inserting the shared zeropage through 
dax_load_hole()->vmf_insert_page_mkwrite() and there is already 
something ... we return VM_FAULT_NOPAGE.

(2) when we try inserting a PTE mapping an ordinary folio through 
dax_fault_iter()->vmf_insert_page_mkwrite() and there is already 
something ... we return VM_FAULT_NOPAGE.

(3) when we try inserting a PMD mapping an ordinary folio through 
dax_fault_iter()->vmf_insert_folio_pmd() and there is already something 
... we return VM_FAULT_NOPAGE.

So that makes me think ... the VM_FAULT_FALLBACK right now is probably 
... wrong? And probably cannot be triggered?

If there is already the huge zerofolio mapped, all good.

Anything else is really not expected I would assume?

-- 
Cheers,

David / dhildenb