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

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Message-ID: <Zp7MaJ0eosfaZP_3@x1n>
Date: Mon, 22 Jul 2024 17:17:28 -0400
From: Peter Xu <peterx@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	linux-kernel@...r.kernel.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,
	Yan Zhao <yan.y.zhao@...el.com>, Kevin Tian <kevin.tian@...el.com>,
	Pei Li <peili.dev@...il.com>, David Hildenbrand <david@...hat.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 04:22:45PM -0400, Liam R. Howlett wrote:
> The problem report from [2] and [3] is that we are getting to a call
> path that includes unmap_single_vma() without the mmap lock.  This patch
> fails to address that issue, it only takes the caller with the assert
> out of the call path.
> 
> Removing the function with the lock check doesn't fix the locking issue.
> If there is no locking issue here, please state the case in the commit
> log as you feel it is safe to use a vma pointer without the mmap_lock
> held.

Could you please state why there's a locking issue, and why this patch (of
a x86 PAT specific issue...) would need any documentation on that?

I thought it was pretty common that file truncation (or anything similar)
does a file rmap walk over vmas that mapping this file, and vmas need to be
alive during the rmap walk, no?

Thanks,

-- 
Peter Xu