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

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Message-ID: <ZpgJD_LAYomCIk4a@x1n>
Date: Wed, 17 Jul 2024 14:10:23 -0400
From: Peter Xu <peterx@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, 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>,
	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 Wed, Jul 17, 2024 at 01:31:54PM -0300, Jason Gunthorpe wrote:
> On Wed, Jul 17, 2024 at 12:30:19PM -0400, Peter Xu wrote:
> > On Wed, Jul 17, 2024 at 04:17:12PM +0200, David Hildenbrand wrote:
> > > I'd be curious how a previous truncation on a file can tear down a PFNMAP
> > > mapping -- and if there are simple ways to forbid that (if possible).
> > 
> > Unfortunately, forbiding it may not work for vfio, as vfio would like to
> > allow none (or partial) mapping on the bars.. at least so far that's the
> > plan.
> 
> vfio doesn't support truncation? Or does that means something else
> here?

Here I meant VFIO can explicitly zapping pgtables via vfio_pci_zap_bars(),
irrelevant of any form of truncations.  I suppose it'll have the same
effect where we may need to stop assuming PFNMAP vmas will always have
fully installed pgtables.

Thanks,

-- 
Peter Xu