linux-kernel - Re: [PATCH] mm/x86/pat: Only untrack the pfn range if unmap region

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Message-ID: <ZpkglfpbHH7lQSft@x1n>
Date: Thu, 18 Jul 2024 10:03:01 -0400
From: Peter Xu <peterx@...hat.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jason Gunthorpe <jgg@...dia.com>, Al Viro <viro@...iv.linux.org.uk>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	"x86@...nel.org" <x86@...nel.org>,
	"Tian, Kevin" <kevin.tian@...el.com>, Pei Li <peili.dev@...il.com>,
	David Hildenbrand <david@...hat.com>, David Wang <00107082@....com>,
	Bert Karwatzki <spasswolf@....de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] mm/x86/pat: Only untrack the pfn range if unmap region

On Thu, Jul 18, 2024 at 09:50:31AM +0800, Yan Zhao wrote:
> Ok. Then if we have two sets of pfns, then we can
> 1. Call remap_pfn_range() in mmap() for pfn set 1.

I don't think this will work..  At least from the current implementation,
remap_pfn_range() will only reserve the memtype if the range covers the
whole vma.

> 2. Export track_pfn_remap() and call track_pfn_remap() in mmap() for pfn
>    set 2.
> 3. Unmap and call vmf_insert_pfn() in the fault handler to map pfn set 2.

IMO this should be the similar case of MMIO being disabled on the bar,
where we can use track_pfn_remap() to register the whole vma in mmap()
first. Then in this case if you prefer proactive injection of partial of
the pfn mappings, one can do that via vmf_insert_pfn*() in mmap() after the
memtype registered.

> 
> However, I'm not sure if we can properly untrack both pfn sets 1 and 2.

Untrack should so far only happen per-vma, AFAIU, so "set 1+2" need to be
done together as they belong to the same vma.

> 
> By exporting untrack_pfn() too? Or, you'll leave VFIO to use ioremap/iounmap()
> (or the variants) to reserve memtype by itself?

Thanks,

-- 
Peter Xu