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

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Message-ID: <897590f7-99a5-4053-8566-76623b929c7c@redhat.com>
Date: Thu, 5 Jun 2025 14:21:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Dan Williams <dan.j.williams@...el.com>
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 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. ;)

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.

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().


Of course, a wild (and different) mixture on pmd_mkdevmap() as well.

So happy to see that go away.

[1] https://lkml.kernel.org/r/20250603211634.2925015-1-david@redhat.com

-- 
Cheers,

David / dhildenb