linux-kernel - Re: [PATCH v2] docs/mm: add VMA locks documentation

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Message-ID: <77ebb384-cfeb-4b13-aaad-c9a6b398fecd@lucifer.local>
Date: Thu, 14 Nov 2024 20:45:38 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>, Alice Ryhl <aliceryhl@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Matthew Wilcox <willy@...radead.org>, Mike Rapoport <rppt@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
        Hillf Danton <hdanton@...a.com>, Qi Zheng <zhengqi.arch@...edance.com>,
        SeongJae Park <sj@...nel.org>, Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v2] docs/mm: add VMA locks documentation

On Tue, Nov 12, 2024 at 10:15:44AM -0500, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241108 08:57]:
> > +VMA locks are special in that you must obtain an mmap **write** lock **first**
> > +in order to obtain a VMA **write** lock. A VMA **read** lock however can be
> > +obtained without any other lock (:c:func:`!lock_vma_under_rcu` will acquire then
> > +release an RCU lock to lookup the VMA for you).
>
> This reduces the impact of a writer on readers by only impacting
> conflicting areas of the vma tree.

Sorry I totally missed this last time round, will add a note.


> > +   ================================= ===================== ======================================== ===============
> > +   Field                             Configuration option  Description                              Write lock
> > +   ================================= ===================== ======================================== ===============
> > +   :c:member:`!anon_name`            CONFIG_ANON_VMA_NAME  A field for storing a                    mmap write,
> > +                                                           :c:struct:`!struct anon_vma_name`        VMA write.
> > +                                                           object providing a name for anonymous
> > +                                                           mappings, or :c:macro:`!NULL` if none
> > +                                                           is set or the VMA is file-backed.
>
> These are ref counted and can be shared by more than one vma for
> scalability.
>
> > +   :c:member:`!swap_readahead_info`  CONFIG_SWAP           Metadata used by the swap mechanism      mmap read,
> > +                                                           to perform readahead. This field is      swap-specific
> > +                                                           accessed atomically.                     lock.
> > +   :c:member:`!vm_policy`            CONFIG_NUMA           :c:type:`!mempolicy` object which        mmap write,
> > +                                                           describes the NUMA behaviour of the      VMA write.
> > +                                                           VMA.
>
> These are also ref counted for scalability.

I think actually on second thoughts I will add notes about this in v3, as it at
least gives a bit more context.