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

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Message-ID: <Zpmi2JWirNDiJz+7@yzhao56-desk.sh.intel.com>
Date: Fri, 19 Jul 2024 07:18:48 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Peter Xu <peterx@...hat.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 10:03:01AM -0400, Peter Xu wrote:
> 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.
Hmm, by referring to pfn set 1 and pfn set 2, I mean that they're both
covering the entire vma, but at different times.

To make it more accurately:

Consider this hypothetical scenario (not the same as what's implemented in
vfio-pci, but seems plausible):

Suppose we have a vma covering only one page, then
(1) Initially, the vma is mapped to pfn1, with remap_pfn_range().
(2) Subsequently, unmap_single_vma() is invoked to unmap the entire VMA.
(3) The driver then maps the entire vma to pfn2 in fault handler

Given this context, my questions are:
1. How can we reserve the memory type for pfn2? Should we call
   track_pfn_remap() in mmap() in advance?
2. How do we untrack the memory type for pfn1 and pfn2, considering they
   belong to the same VMA but mutual exclusively and not concurrently?

Thanks
Yan
> 
> > 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
>