linux-ext4 - Re: [PATCH 07/12] mm: Remove redundant pXd

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Message-ID: <79f75185-ac45-47c6-8f56-2ffa1e4815b0@redhat.com>
Date: Thu, 5 Jun 2025 19:04:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Dan Williams <dan.j.williams@...el.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
 gerald.schaefer@...ux.ibm.com, willy@...radead.org,
 linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org,
 balbirs@...dia.com, lorenzo.stoakes@...cle.com,
 linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org, John@...ves.net
Subject: Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls

On 05.06.25 18:30, Dan Williams wrote:
> David Hildenbrand wrote:
>> On 05.06.25 14:09, Jason Gunthorpe wrote:
>>> On Wed, Jun 04, 2025 at 07:35:24PM -0700, Dan Williams wrote:
>>>
>>>> If all dax pages are special, then vm_normal_page() should never find
>>>> them and gup should fail.
>>>>
>>>> ...oh, but vm_normal_page_p[mu]d() is not used in the gup path, and
>>>> 'special' is not set in the pte path.
>>>
>>> That seems really suboptimal?? Why would pmd and pte be different?
>>>
>>>> I think for any p[mu]d where p[mu]d_page() is ok to use should never set
>>>> 'special', right?
>>>
>>> There should be dedicated functions for installing pages and PFNs,
>>> only the PFN one would set the special bit.
>>>
>>> And certainly your tests *should* be failing as special entries should
>>> never ever be converted to struct page.
>>
>> Worth reviewing [1] where I clean that up and describe the current
>> impact. ;)
> 
> Will do.
> 
>> What's even worse about this pte_devmap()/pmd_devmap()/... shit (sorry!
>> but it's absolute shit) is that some pte_mkdev() set the pte special,
>> while others ... don't.
> 
> As the person who started the turd rolling into this pile that Alistair
> is heroically cleaning up, I approve this characterization.
> 
>> E.g., loongarch
>>
>> static inline pte_t pte_mkdevmap(pte_t pte)	{ pte_val(pte) |=
>> _PAGE_DEVMAP; return pte; }
>>
>> I don't even know how it can (could) survive vm_normal_page().
> 
> Presently "can" because dax switched away from vmf_insert_mixed() to
> vmf_insert_page(), "could" in the past was the devmap hack to avoid
> treating VM_MIXEDMAP as !vm_normal_page().

The thing is, in vm_normal_page() if we have CONFIG_ARCH_HAS_PTE_SPECIAL 
-- which loongarch sets -- if we don't see pte_special(), we will assume 
that it is refcounted.

	if (likely(!pte_special(pte))
		goto check_pfn;

So if pte_mkdevmap() does not set pte_special(), then ... 
vm_normal_page() would detect it as normal, although it isn't normal?

But maybe I am missing something important.

-- 
Cheers,

David / dhildenb