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

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Message-ID: <20240723214412.GQ3371438@nvidia.com>
Date: Tue, 23 Jul 2024 18:44:12 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Peter Xu <peterx@...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 Tue, Jul 23, 2024 at 11:36:51PM +0200, David Hildenbrand wrote:
> I wonder if we could just let relevant users do the PAT handling manually:
> I'm also not sure how many remap_pfn_range users end up triggering VM_PAT
> code although they don't really have to (just because they happen to cover a
> full VMA)?
> 
> One nasty thing is fork(), I was wondering if relevant users really rely on
> that or if we could force these VMAs to simply not get copied during fork.
> During fork() we have to "duplicate" the reservation ...

I admit I barely understand what x86 uses this PAT stuff for -
allowing WC mappings is part of it?

If yes, then RDMA would expect WC PFN MAP VMAs to copy their WCness
during fork..

Jason