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

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Message-ID: <50a581c9-0e59-4282-859b-922d02b3870e@redhat.com>
Date: Tue, 23 Jul 2024 12:12:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, "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 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 22.07.24 23:17, Peter Xu wrote:
> 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?

Right, I was also assuming that the rmap locking (from where we obtained 
that VMA -- rmap interval tree) makes sure that using the VMA is safe.

-- 
Cheers,

David / dhildenb