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

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Message-ID: <Zp4A6HNhamaNQJOL@yzhao56-desk.sh.intel.com>
Date: Mon, 22 Jul 2024 14:49:12 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Peter Xu <peterx@...hat.com>
CC: David Hildenbrand <david@...hat.com>, "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 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 Fri, Jul 19, 2024 at 10:13:33AM -0400, Peter Xu wrote:
> On Fri, Jul 19, 2024 at 10:28:09AM +0200, David Hildenbrand wrote:
> > On 19.07.24 01:18, Yan Zhao wrote:
> > > 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?
> > 
> > Do we really have to support such changing PFNs in a VMA? Are there existing
> > use cases that would rely on that?
> 
> I share the same question with David.  I don't think we support that, and I
> don't know whether we should, either.
> 
> Such flexibility already will break with current PAT design.  See:
Previously with remap_pfn_range() being able to be called in fault handlers,
this flexibility is doable. i.e. reserve in the fault handler and untrack
in unmap_single_vma().

> 
> untrack_pfn:
> 	if (!paddr && !size) {
> 		if (get_pat_info(vma, &paddr, NULL))
> 			return;
> 		size = vma->vm_end - vma->vm_start;
> 	}
> 	free_pfn_range(paddr, size);  <---- assumes PFNs to be continuous
> 
> So untrack_pfn() already assumed the pfn being continuous.  I think it
> means pfns cannot be randomly faulted in, but determined when mmap().
Hmm, in the hypothetical scenario, (vma->vm_end - vma->vm_start) == PAGE_SIZE.