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

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Message-ID: <1a0884cb-39ed-455e-a505-7c1b2a0e5225@redhat.com>
Date: Fri, 19 Jul 2024 10:28:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Yan Zhao <yan.y.zhao@...el.com>, 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 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 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?

Would it be a problem if we would merge the mmap+track_pfn_remap, such 
that such a special VMA can only ever belong to a single PFN range, that 
is fixed, but PFNs can be faulted in lazily?

-- 
Cheers,

David / dhildenb