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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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@oracle.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.

>
> Alice