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

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Message-ID: <CAH5fLgg-f6LvCTejO6f-04cQTRNC4rE8AwnQdN1NGuN_W2HxNg@mail.gmail.com>
Date: Tue, 14 Jan 2025 14:42:05 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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!

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


Alice