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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Lorenzo Stoakes" <lorenzo.stoakes@oracle.com> writes:

> On Thu, Jan 09, 2025 at 10:50:13AM +0100, Andreas Hindborg wrote:
>> "Lorenzo Stoakes" <lorenzo.stoakes@oracle.com> writes:
>>
>> > On Thu, Jan 09, 2025 at 09:02:11AM +0100, Andreas Hindborg wrote:
>> >> "Alice Ryhl" <aliceryhl@google.com> writes:
>> >>
>> >> > On Mon, Dec 16, 2024 at 3:51 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>> >> >>
>> >> >>
>> >> >> > +
>> >> >> > +    /// Zap pages in the given page range.
>> >> >> > +    ///
>> >> >> > +    /// This clears page table mappings for the range at the leaf level, leaving all other page
>> >> >> > +    /// tables intact,
>> >> >>
>> >> >> I don't fully understand this docstring. Is it correct that the function
>> >> >> will unmap the address range given by `start` and `size`, _and_ free the
>> >> >> pages used to hold the mappings at the leaf level of the page table?
>> >> >
>> >> > If the vma owns a refcount on those pages, then the refcounts are dropped.
>> >>
>> >> Maybe drop the "at the leaf level leaving all other page tables intact".
>> >> It confuses me, since when would this not be the case?
>> >
>> > I don't understand your objection. The whole nature of a zap is to traverse
>> > leaf level page table mappings, clearing the entries, leaving the other
>> > page table entries intact.
>>
>> As someone not deeply familiar with this function and it's use, I became
>> uncertain of my understanding when I read this sentence. As I asked
>> above: When would you not clear mappings at the leaf level and leave all
>> other mappings alone?
>
> Because these are page tables and page tables can span multiple PTE
> tables. Correctly removing at the time of clearing would be expensive and
> require very careful handling.

What is the distinction between clearing a PTE and removing it?

I asked above if the leaf page holding the PTEs would be dropped if all
the PTEs it holds are cleared. Alice "If the vma owns a refcount on those pages,
then the refcounts are dropped.". But from your message I am guessing
maybe not?

>
>>
>> Imagine you have a collection structure backed by a tree and the
>> `remove_item` function has the sentence "remove item at the leaf level
>> but leave all other items in the collection alone". That would be over
>> specifying. It is enough information in the user facing documentation
>> that the item is removed. You don't need to state that a remove
>> operation on an item does not remove other items. Does this example
>> transfer to this function, or am I missing something?
>
> No, because we're dealing with page tables and you are explicitly requesting a
> page table operation. Knowing what is touched is meaningful.

When would a page table operation to remove (clear?) the PTEs
corresponding to an address range touch PTEs corresponding to addresses
outside of the range?

>
>>
>> > That is, precisely what is written here. In fact I think this
>> > characterisation is derived from discussions had with us in mm, and it is
>> > one with which I am happy.
>> >
>> > Why is it problematic to accurately describe what this does?
>>
>> Again, it might be that I don't properly understand what the function
>> actually does, but if it is just removing the entries described by the
>> range - write that. Don't add irrelevant details or specify what the
>> function does not do. It slows down the user when reading documentation.
>
> It is highly pertinent as mentioned above.
>
> I mean we can expand the comment to explicitly add some detail around this
> since obviously this is confusing (hey - a lot of mm is confusing - this is
> an ongonig problem and why I have gone to lengths to try to improve
> documentation and wrote a book about it :)

That would be nice :)

>
>>
>> >
>> > 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.

>
>>
>> >
>> >>
>> >> >
>> >> >> > and freeing any memory referenced by the VMA 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.
>> >> >> > +    #[inline]
>> >> >> > +    pub fn zap_page_range_single(&self, address: usize, size: usize) {
>> >>
>> >>
>> >> Best regards,
>> >> Andreas Hindborg
>> >>
>> >>
>> >
>> > Let's please get this series merged. I think Alice has demonstrated
>> > remarkable patience already, and modulo significant technical pushback on
>> > the rust side (on which I defer entirely to the expertise of rust people),
>> > I want to see this go in.
>>
>> I am sensing that you don't feel my comments are relevant at the current
>> stage of this series (v11). Alice asked for reviews of the series. These are my
>> comments. Feel free do whatever you want with them.
>
> I think you're getting the wrong end of the stick - you are making comments
> on something relevant to mm, as an mm maintainer I'm giving you my point of
> view.

I appreciate that.

>
> Your comments elsewhere seem highly useful, and review is always
> appreciated, if you read what I said above - I defer entirely to the rust
> community on things of which you are expert - so there is clearly no
> disrespect intended.

I did not read any disrespect in your message. I understand if you think
I am late at the party at v11. Normally I would not pick up review of a
series that late.

>
> I'd also ask you to respect that I have gone to great lengths to review
> this series from mm side, motivated by a strong desire to help the rust
> commnuity.

I absolutely appreciate that!

>
> So where I am coming from is nothing negative, quite the opposite, I simply
> feel _on this issue_ it is not worth holding up the series for.
>
> This is no way intended to do down, disrespect or seem ungrateful for your
> review or efforts. Apologies if it seemed that way, was not the intent.
>
> And to reiterate what I said above - I want to see this series merge :) so
> there is no ill will anywhere.

We can always merge this as is and then discuss the finer points of
documentation later - I am fine with that. But obviously I cannot put my
review tag on it, when I don't understand the semantics of the functions
from reading the documentation strings. Perhaps we have someone who is
more well versed in mm that can.

>
>>
>>
>> Best regards,
>> Andreas Hindborg
>>
>
> Perhaps the correct approach here, as alluded above, is for Alice to add an
> extra commentary pointing out the role of page tables here?

That would be nice. Perhaps a bit of module level documentation is also
a good addition.

>
> To complicate matters further (of course) there are recent series which
> actually _do_ unused clean up page tables, though not (I believe... I have
> to check...) on zap. But of course we in mm JUST LOVE to complicate
> everything... ;)

We should make sure to document that :)


Best regards,
Andreas Hindborg