linux-kernel - Re: [PATCH v7 4/4] userfaultfd: use per-vma locks in userfaultfd operations

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Message-ID: <7a4f8c38-13a1-4a28-b7ce-ad3bb983dd69@redhat.com>
Date: Thu, 23 Jan 2025 18:14:50 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Barry Song <21cnbao@...il.com>
Cc: lokeshgidra@...gle.com, Liam.Howlett@...cle.com, aarcange@...hat.com,
 akpm@...ux-foundation.org, axelrasmussen@...gle.com, bgeffon@...gle.com,
 jannh@...gle.com, kaleshsingh@...gle.com, kernel-team@...roid.com,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, ngeoffray@...gle.com, peterx@...hat.com,
 rppt@...nel.org, ryan.roberts@....com, selinux@...r.kernel.org,
 surenb@...gle.com, timmurray@...gle.com, willy@...radead.org
Subject: Re: [PATCH v7 4/4] userfaultfd: use per-vma locks in userfaultfd
 operations

>>
>>           ┌────────────┐TASK_SIZE
>>           │            │
>>           │            │
>>           │            │mmap VOLATILE
>>           ┼────────────┤
>>           │            │
>>           │            │
>>           │            │
>>           │            │
>>           │            │default mmap
>>           │            │
>>           │            │
>>           └────────────┘
>>
>> VMAs in the volatile region are assigned their own volatile_mmap_lock,
>> which is independent of the mmap_lock for the non-volatile region.
>> Additionally, we ensure that no single VMA spans the boundary between
>> the volatile and non-volatile regions. This separation prevents the
>> frequent modifications of a small number of volatile VMAs from blocking
>> other operations on a large number of non-volatile VMAs.
> 
> I think really overall this will be solving one can of worms by introducing
> another can of very large worms in space :P but perhaps I am missing
> details here.

Fully agreed; not a big fan :)

-- 
Cheers,

David / dhildenb