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

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Message-ID: <6841c63b3cb25_249110060@dwillia2-xfh.jf.intel.com.notmuch>
Date: Thu, 5 Jun 2025 09:30:51 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: David Hildenbrand <david@...hat.com>, 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

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().