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

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Message-ID: <CAH5fLgiJrLt7jr9nPDvVps1PeO_53wVtjOvQ2a8dwjSo7=vchA@mail.gmail.com>
Date: Wed, 15 Jan 2025 12:04:57 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 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 Wed, Jan 15, 2025 at 12:03 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com> writes:
>
> > 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.
>
> Sounds good to me - thanks.
>
> @Alice - can we add PTE, PTE entry, PMD, PUD to the vocabulary at the
> top? Not sure if it should go here in virt.rs or in mm.rs. If you have
> no cycles I can try to add it down the road.

Sorry, I don't know these concepts well enough to write about them
right now. If there's a documentation page I can link to that's great,
but otherwise I think it will have to wait.

Alice