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

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Message-ID: <CAJuCfpF2iCo+ZKrqYam6wjqn7LYu4cnDeqDKrd-LpHerc5WHVw@mail.gmail.com>
Date: Thu, 23 Jan 2025 09:46:30 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Barry Song <21cnbao@...il.com>, 
	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, timmurray@...gle.com, 
	willy@...radead.org
Subject: Re: [PATCH v7 4/4] userfaultfd: use per-vma locks in userfaultfd operations

On Thu, Jan 23, 2025 at 9:15 AM David Hildenbrand <david@...hat.com> wrote:
>
> >>
> >>           ┌────────────┐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 :)

+1. Let's not add more coarse-grained locks in mm. Discussing this at
LSFMM as Liam suggested would be a good idea. I'm definitely
interested.

>
> --
> Cheers,
>
> David / dhildenb
>