linux-kernel - Re: [PATCH v11 2/8] mm: rust: add vm_area_struct methods that require read access

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Message-ID: <cb8d8595-5a74-42f3-821e-75cd0bbfbfbe@lucifer.local>
Date: Tue, 14 Jan 2025 15:33:57 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>,
        Jann Horn <jannh@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>,
        Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v11 2/8] mm: rust: add vm_area_struct methods that
 require read access

On Tue, Jan 14, 2025 at 02:42:05PM +0100, Alice Ryhl wrote:
> On Tue, Jan 14, 2025 at 12:57 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > On Tue, Jan 14, 2025 at 10:50:01AM +0100, Alice Ryhl wrote:
> > > On Mon, Jan 13, 2025 at 3:45 PM Lorenzo Stoakes
> > > <lorenzo.stoakes@...cle.com> wrote:
> > > > > >> > For a series at v11 where there is broad agreement with maintainers within
> > > > > >> > the subsystem which it wraps, perhaps the priority should be to try to have
> > > > > >> > the series merged unless there is significant technical objection from the
> > > > > >> > rust side?
> > > > > >> >
> > > > > >> >>
> > > > > >> >> How about this:
> > > > > >> >>
> > > > > >> >> This clears the virtual memory map for the range given by `start` and
> > > > > >> >> `size`, dropping refcounts to memory held by the mappings in this range. That
> > > > > >> >> is, anonymous memory is completely freed, file-backed memory has its
> > > > > >> >> reference count on page cache folio's dropped, any dirty data will still
> > > > > >> >> be written back to disk as usual.
> > > > > >> >
> > > > > >> > Sorry I object to this, 'clears the virtual memory map' is really
> > > > > >> > vague. What is already there is better.
> > > > > >>
> > > > > >> Would you like the proposed paragraph if we replaced "virtual memory
> > > > > >> map" with "page table mappings", or do you object to the entirety of the
> > > > > >> new suggestion?
> > > > > >
> > > > > > I object to the suggestion in general. The description is fine as it is.
> > > > >
> > > > > Ok. I'm raising a flag because I had more questions after reading the
> > > > > docstring than before.
> > > >
> > > > Sure and so I think this is valuable information, and indicates it's
> > > > probably worthwhile adding a little extra information on mentioning page
> > > > tables.
> > >
> > > Sorry, I'm a bit lost. What would you like me to add? Perhaps there's
> > > an existing file in Documentation/ that I can link to?
> >
> > Sure no problem, I propose expanding:
> >
> > /// This clears page table mappings for the range at the leaf level, leaving all other page
> > /// tables intact,
> > /// anonymous memory is completely freed, file-backed memory has its reference count on page
> > /// cache folio's dropped, any dirty data will still be written back to disk as usual.
> >
> > To include information on page tables. I suggest something like:
> >
> > /// It may seem odd that we clear at the leaf level, this is however a product
> > /// of the page table structure used to map physical memory into a virtual
> > /// address space - each virtual address actually consists of a bitmap of array
> > /// indices into page tables, which form a hierarchical page table level
> > /// structure.
> > ///
> > /// As a result, each page table level maps a multiple of page table levels
> > /// below, and thus span ever increasing ranges of pages. At the leaf or PTE
> > /// level, we map the actual physical memory.
> > ///
> > /// It is here where a zap operates, as it the only place we can be certain of
> > /// clearing without impacting any other virtual mappings. It is an
> > /// implementation detail as to whether the kernel goes further in freeing
> > /// unused page tables, but for the purposes of this operation we must only
> > /// assume that the leaf level is cleared.
> >
> > Alice, Andreas - please let me know if this makes sense/is clear or needs
> > further clarification.
>
> That looks reasonable to me. Thanks!

Cool!

>
> Do you have thoughts on the wordings I proposed here?
> https://lore.kernel.org/all/CAH5fLginc=uNPVp1-T-oBrgtE1oi_cBMd65sPkDgqSDjH_CNfA@mail.gmail.com/

Oops, missed that. Do always feel free to ping me if I seem to miss things!

Will reply in thread

>
>
> Alice