linux-kernel - Re: Race condition observed between page migration and page fault handling on arm64 machines

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Message-ID: <bbe411f2-4c68-4f92-af8c-da184669dca8@arm.com>
Date: Thu, 1 Aug 2024 15:08:30 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
 willy@...radead.org
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
 cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 osalvador@...e.de, baolin.wang@...ux.alibaba.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 ioworker0@...il.com, gshan@...hat.com, mark.rutland@....com,
 kirill.shutemov@...ux.intel.com, hughd@...gle.com, aneesh.kumar@...nel.org,
 yang@...amperecomputing.com, peterx@...hat.com, broonie@...nel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: Race condition observed between page migration and page fault
 handling on arm64 machines


On 8/1/24 14:12, David Hildenbrand wrote:
> On 01.08.24 10:16, Dev Jain wrote:
>> I and Ryan had a discussion and we thought it would be best to get 
>> feedback
>> from the community.
>>
>> The migration mm selftest currently fails on arm64 for shared anon 
>> mappings,
>> due to the following race:
>
> Do you mean MAP_SHARED|MAP_ANON or MAP_PRIVATE|MAP_ANON_fork? Because 
> you note shmem below, I assume you mean MAP_SHARED|MAP_ANON


Yes.

>
>>
>> Migration:                        Page fault:
>> try_to_migrate_one():                    handle_pte_fault():
>> 1. Nuke the PTE                        PTE has been deleted => 
>> do_pte_missing()
>> 2. Mark the PTE for migration                PTE has not been deleted 
>> but is just not "present" => do_swap_page()
>>
>
> In filemap_fault_recheck_pte_none() we recheck under PTL to make sure 
> that a temporary pte_none() really was persistent pte_none() and not a 
> temporary pte_none() under PTL.
>
> Should we do something similar in do_fault()? I see that we already do 
> something like that on the "!vma->vm_ops->fault" path.
>
> But of course, there is a tradeoff between letting migration 
> (temporarily) fail and grabbing the PTL during page faults.


To dampen the tradeoff, we could do this in shmem_fault() instead? But 
then, this would mean that we do this in all

kinds of vma->vm_ops->fault, only when we discover another reference 
count race condition :) Doing this in do_fault()

should solve this once and for all. In fact, do_pte_missing() may call 
do_anonymous_page() or do_fault(), and I just

noticed that the former already checks this using vmf_pte_changed().