implement the equivalent of the rust std's Vec::resize
on the kernel's Vec type.

Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
---
 rust/kernel/alloc/kvec.rs | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 18bcc59f0b38..eb6d40a1bf8b 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -554,6 +554,32 @@ pub fn from_elem(value: T, n: usize, flags: Flags) -> Result<Self, AllocError> {
 
         Ok(v)
     }
+
+    /// Resizes the [`Vec`] so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is smaller than `len`, the `Vec` is [`Vec::truncate`]d.
+    /// If `new_len` is larger, each new slot is filled with clones of `value`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    /// v.resize(1, 42, GFP_KERNEL)?;
+    /// assert_eq!(&v, &[1]);
+    ///
+    /// v.resize(3, 42, GFP_KERNEL)?;
+    /// assert_eq!(&v, &[1, 42, 42]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
+        if new_len > self.len() {
+            self.extend_with(new_len - self.len(), value, flags)
+        } else {
+            self.truncate(new_len);
+            Ok(())
+        }
+    }
 }
 
 impl<T, A> Drop for Vec<T, A>
-- 
2.48.1

On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
<andrewjballance@gmail.com> wrote:
>
> implement the equivalent of the rust std's Vec::resize
> on the kernel's Vec type.
>
> Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> ---
>  rust/kernel/alloc/kvec.rs | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 18bcc59f0b38..eb6d40a1bf8b 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -554,6 +554,32 @@ pub fn from_elem(value: T, n: usize, flags: Flags) -> Result<Self, AllocError> {
>
>          Ok(v)
>      }
> +
> +    /// Resizes the [`Vec`] so that `len` is equal to `new_len`.
> +    ///
> +    /// If `new_len` is smaller than `len`, the `Vec` is [`Vec::truncate`]d.
> +    /// If `new_len` is larger, each new slot is filled with clones of `value`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut v = kernel::kvec![1, 2, 3]?;
> +    /// v.resize(1, 42, GFP_KERNEL)?;
> +    /// assert_eq!(&v, &[1]);
> +    ///
> +    /// v.resize(3, 42, GFP_KERNEL)?;
> +    /// assert_eq!(&v, &[1, 42, 42]);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> +        if new_len > self.len() {
> +            self.extend_with(new_len - self.len(), value, flags)
> +        } else {
> +            self.truncate(new_len);
> +            Ok(())
> +        }
> +    }

You can avoid underflow checking in debug builds by using `checked_sub`:

        match new_len.checked_sub(self.len()) {
            Some(n) => self.extend_with(n, value, flags),
            None => {
                self.truncate(new_len);
                Ok(())
            }
        }

>  }
>
>  impl<T, A> Drop for Vec<T, A>
> --
> 2.48.1
>
>

Either way:

Reviewed-by: Tamir Duberstein <tamird@gmail.com>

On Tue Mar 18, 2025 at 9:12 PM CET, Tamir Duberstein wrote:
> On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
> <andrewjballance@gmail.com> wrote:
>> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
>> +        if new_len > self.len() {
>> +            self.extend_with(new_len - self.len(), value, flags)
>> +        } else {
>> +            self.truncate(new_len);
>> +            Ok(())
>> +        }
>> +    }
>
> You can avoid underflow checking in debug builds by using `checked_sub`:

`checked_sub` doesn't only avoid underflow in debug builds, but rather
in all builds. But the code below is a good suggestion.

---
Cheers,
Benno

>         match new_len.checked_sub(self.len()) {
>             Some(n) => self.extend_with(n, value, flags),
>             None => {
>                 self.truncate(new_len);
>                 Ok(())
>             }
>         }
>
>>  }
>>
>>  impl<T, A> Drop for Vec<T, A>
>> --
>> 2.48.1
>>
>>
>
> Either way:
>
> Reviewed-by: Tamir Duberstein <tamird@gmail.com>

On Tue, Mar 18, 2025 at 8:50 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Tue Mar 18, 2025 at 9:12 PM CET, Tamir Duberstein wrote:
> > On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
> > <andrewjballance@gmail.com> wrote:
> >> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> >> +        if new_len > self.len() {
> >> +            self.extend_with(new_len - self.len(), value, flags)
> >> +        } else {
> >> +            self.truncate(new_len);
> >> +            Ok(())
> >> +        }
> >> +    }
> >
> > You can avoid underflow checking in debug builds by using `checked_sub`:
>
> `checked_sub` doesn't only avoid underflow in debug builds, but rather
> in all builds. But the code below is a good suggestion.

Yes, I know :)

I included that language because the underflow check is likely
optimized away in release builds.

Tamir

On Wed Mar 19, 2025 at 2:42 PM CET, Tamir Duberstein wrote:
> On Tue, Mar 18, 2025 at 8:50 PM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> On Tue Mar 18, 2025 at 9:12 PM CET, Tamir Duberstein wrote:
>> > On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
>> > <andrewjballance@gmail.com> wrote:
>> >> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
>> >> +        if new_len > self.len() {
>> >> +            self.extend_with(new_len - self.len(), value, flags)
>> >> +        } else {
>> >> +            self.truncate(new_len);
>> >> +            Ok(())
>> >> +        }
>> >> +    }
>> >
>> > You can avoid underflow checking in debug builds by using `checked_sub`:
>>
>> `checked_sub` doesn't only avoid underflow in debug builds, but rather
>> in all builds. But the code below is a good suggestion.
>
> Yes, I know :)
>
> I included that language because the underflow check is likely
> optimized away in release builds.

If the function is inlined and the compiler can argue that `new_len >
self.len()`, then yes, but otherwise I'm pretty sure it won't be
optimized away.

Also if it is optimized away, then the check was still "executed", so I
find it a bit misleading to say "in debug builds" (making it sound like
it wouldn't do it in non-debug builds).

---
Cheers,
Benno

On Wed, Mar 19, 2025 at 10:34 AM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On Wed Mar 19, 2025 at 2:42 PM CET, Tamir Duberstein wrote:
> > On Tue, Mar 18, 2025 at 8:50 PM Benno Lossin <benno.lossin@proton.me> wrote:
> >>
> >> On Tue Mar 18, 2025 at 9:12 PM CET, Tamir Duberstein wrote:
> >> > On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
> >> > <andrewjballance@gmail.com> wrote:
> >> >> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> >> >> +        if new_len > self.len() {
> >> >> +            self.extend_with(new_len - self.len(), value, flags)
> >> >> +        } else {
> >> >> +            self.truncate(new_len);
> >> >> +            Ok(())
> >> >> +        }
> >> >> +    }
> >> >
> >> > You can avoid underflow checking in debug builds by using `checked_sub`:
> >>
> >> `checked_sub` doesn't only avoid underflow in debug builds, but rather
> >> in all builds. But the code below is a good suggestion.
> >
> > Yes, I know :)
> >
> > I included that language because the underflow check is likely
> > optimized away in release builds.
>
> If the function is inlined and the compiler can argue that `new_len >
> self.len()`, then yes, but otherwise I'm pretty sure it won't be
> optimized away.
>
> Also if it is optimized away, then the check was still "executed", so I
> find it a bit misleading to say "in debug builds" (making it sound like
> it wouldn't do it in non-debug builds).

If we're talking about the same thing then I think we're both wrong
and the correct phrasing would have been: "you can avoid underflow
checking when CONFIG_RUST_OVERFLOW_CHECKS=y by using `checked_sub`". I
was referring to the underflow check implicit in `new_len -
self.len()`.

On Wed, Mar 19, 2025 at 4:59 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> If we're talking about the same thing then I think we're both wrong
> and the correct phrasing would have been: "you can avoid underflow
> checking when CONFIG_RUST_OVERFLOW_CHECKS=y by using `checked_sub`". I
> was referring to the underflow check implicit in `new_len -
> self.len()`.

`checked_sub` always checks (if not optimized away). The config option
is about the implicit one.

Do you mean avoiding panics?

Cheers,
Miguel

On Wed, Mar 19, 2025 at 12:06 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Wed, Mar 19, 2025 at 4:59 PM Tamir Duberstein <tamird@gmail.com> wrote:
> >
> > If we're talking about the same thing then I think we're both wrong
> > and the correct phrasing would have been: "you can avoid underflow
> > checking when CONFIG_RUST_OVERFLOW_CHECKS=y by using `checked_sub`". I
> > was referring to the underflow check implicit in `new_len -
> > self.len()`.
>
> `checked_sub` always checks (if not optimized away). The config option
> is about the implicit one.
>
> Do you mean avoiding panics?

No, I meant avoiding the check. The existing code already explicitly
checks `new_len > self.len()` before evaluating `new_len -
self.len()`. This means the check occurs twice. `checked_sub` reduces
the number of checks by 1. Perhaps my wording could have been clearer
("avoid *an* underflow check").

Tamir

On Wed, Mar 19, 2025 at 5:13 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> No, I meant avoiding the check. The existing code already explicitly
> checks `new_len > self.len()` before evaluating `new_len -
> self.len()`. This means the check occurs twice. `checked_sub` reduces
> the number of checks by 1. Perhaps my wording could have been clearer
> ("avoid *an* underflow check").

Ah, you mean in the function you suggested, I see.

I think it they all may compile down to the same thing, whether
overflows checks are enabled or not, and whether the version in the
patch or `checked_sub` is used or not. At least in a quick Compiler
Explorer test it seems so, but I didn't check in an actual kernel
build.

The implicit check is gated behind the other one, so that one can be
removed, even if values are unknown -- we always have optimizations
enabled, even under "debug" builds (assuming "debug" means overflow
checking enabled).

Cheers,
Miguel

On Wed, Mar 19, 2025 at 12:43 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Wed, Mar 19, 2025 at 5:13 PM Tamir Duberstein <tamird@gmail.com> wrote:
> >
> > No, I meant avoiding the check. The existing code already explicitly
> > checks `new_len > self.len()` before evaluating `new_len -
> > self.len()`. This means the check occurs twice. `checked_sub` reduces
> > the number of checks by 1. Perhaps my wording could have been clearer
> > ("avoid *an* underflow check").
>
> Ah, you mean in the function you suggested, I see.
>
> I think it they all may compile down to the same thing, whether
> overflows checks are enabled or not, and whether the version in the
> patch or `checked_sub` is used or not. At least in a quick Compiler
> Explorer test it seems so, but I didn't check in an actual kernel
> build.
>
> The implicit check is gated behind the other one, so that one can be
> removed, even if values are unknown -- we always have optimizations
> enabled, even under "debug" builds (assuming "debug" means overflow
> checking enabled).

Yeah, this elision is what I was trying to allude to in the original
wording. I still think the suggested code is simpler, but gave my RB
either way.

On Sun Mar 16, 2025 at 12:16 PM CET, Andrew Ballance wrote:
> implement the equivalent of the rust std's Vec::resize
> on the kernel's Vec type.
>
> Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

---
Cheers,
Benno

> ---
>  rust/kernel/alloc/kvec.rs | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)