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

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Message-ID: <ZqASQCobvpB_VfCL@x1n>
Date: Tue, 23 Jul 2024 16:27:44 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Yan Zhao <yan.y.zhao@...el.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 Mon, Jul 22, 2024 at 11:17:57AM +0200, David Hildenbrand wrote:
> What we do have is a single VMA, whereby within that VMA we place various
> different PFN ranges. (randomly looking at drivers/video/fbdev/smscufx.c)
> 
> These wouldn't have triggered VM_PAT code.

Right, it looks like VM_PAT is only applying to a whole-vma mapping, even
though I don't know how that was designed..

I wished vma->vm_pgoff was for storing the base PFN for VM_SHARED too: now
it only works like that for CoW mappings in remap_pfn_range_notrack(), then
it looks like VM_SHARED users of remap_pfn_range() can reuse vm_pgoff, and
I think VFIO does reuse it at least..

I am a bit confused on why Linux made that different for VM_SHARED,
probably since b3b9c2932c32 ("mm, x86, pat: rework linear pfn-mmap
tracking").  I wished vm_pgoff was always used for internal maintenance
(even for VM_SHARED) so this issue should be easier to solve.

Maybe we can still re-define vm_pgoff for VM_SHARED pfnmaps? The caller
should always be able to encode information in vm_private_data anyway.
But I think that might break OOT users..

-- 
Peter Xu