rust/kernel/devres.rs | 55 ++++++++++++++++++++++++++++++++++++++++--- rust/kernel/lib.rs | 1 + 2 files changed, 53 insertions(+), 3 deletions(-)
The unconditional 128-byte const array allocation for every unique
`Devres<T>` caused unnecessary .rodata bloat in production builds,
and type names exceeding 127 bytes were silently truncated without
any indication in debug logs.
Gate the `TYPE_NAME`, `TYPE_NAME_BUF`, and `TYPE_NAME_CSTR` constants
behind `#[cfg(CONFIG_DEBUG_DEVRES)]` so the 128-byte buffer is only
allocated when device resource debugging is enabled. For production
builds, use `crate::c_str!("")` as a zero-cost empty C-string fallback.
When the type name is too long to fit in the 127-byte buffer (plus null
terminator), the copy routine now appends "..." at the end — copying
only the first 124 bytes — to clearly indicate truncation rather than
silently dropping trailing characters.
Signed-off-by: Aary Milind Kinge <kingeaary@gmail.com>
---
rust/kernel/devres.rs | 55 ++++++++++++++++++++++++++++++++++++++++---
rust/kernel/lib.rs | 1 +
2 files changed, 53 insertions(+), 3 deletions(-)
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 9e5f93aed20c..79cfe7d9a415 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -185,6 +185,57 @@ pub(super) unsafe fn devres_node_remove(
}
impl<T: Send> Devres<T> {
+ #[cfg(CONFIG_DEBUG_DEVRES)]
+ const TYPE_NAME: &'static str = core::any::type_name::<T>();
+
+ #[cfg(CONFIG_DEBUG_DEVRES)]
+ const TYPE_NAME_BUF: [u8; 128] = {
+ let bytes = Self::TYPE_NAME.as_bytes();
+ let mut buf = [0u8; 128];
+ let mut i = 0;
+
+ if bytes.len() > 127 {
+ // Copy exactly 124 bytes, then append '...' to clearly indicate truncation
+ while i < 124 {
+ buf[i] = bytes[i];
+ i += 1;
+ }
+ buf[124] = b'.';
+ buf[125] = b'.';
+ buf[126] = b'.';
+ buf[127] = 0; // Null terminator
+ } else {
+ // Copy normally
+ while i < bytes.len() {
+ buf[i] = bytes[i];
+ i += 1;
+ }
+ buf[i] = 0; // Null terminator
+ }
+ buf
+ };
+
+ #[cfg(CONFIG_DEBUG_DEVRES)]
+ const TYPE_NAME_CSTR: &'static crate::str::CStr = {
+ let static_buf: &'static [u8; 128] = &Self::TYPE_NAME_BUF;
+
+ let mut len = 0;
+ while len < 128 && static_buf[len] != 0 {
+ len += 1;
+ }
+
+ // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
+ unsafe {
+ crate::str::CStr::from_bytes_with_nul_unchecked(core::slice::from_raw_parts(
+ static_buf.as_ptr(),
+ len + 1,
+ ))
+ }
+ };
+
+ #[cfg(not(CONFIG_DEBUG_DEVRES))]
+ const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
+
/// Creates a new [`Devres`] instance of the given `data`.
///
/// The `data` encapsulated within the returned `Devres` instance' `data` will be
@@ -209,9 +260,7 @@ pub fn new<E>(dev: &Device<Bound>, data: impl PinInit<T, E>) -> Result<Self>
unsafe {
base::devres_set_node_dbginfo(
node,
- // TODO: Use `core::any::type_name::<T>()` once it is a `const fn`,
- // such that we can convert the `&str` to a `&CStr` at compile-time.
- c"Devres<T>".as_char_ptr(),
+ Self::TYPE_NAME_CSTR.as_char_ptr(),
core::mem::size_of::<Revocable<T>>(),
)
};
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index b72b2fbe046d..48df181788f6 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -22,6 +22,7 @@
// Expected to become stable.
#![feature(arbitrary_self_types)]
#![feature(derive_coerce_pointee)]
+#![feature(const_type_name)]
//
// To be determined.
#![feature(used_with_arg)]
--
2.51.0
On Tue, May 26, 2026 at 05:28:25PM +0530, Aary Milind Kinge wrote:
> The unconditional 128-byte const array allocation for every unique
> `Devres<T>` caused unnecessary .rodata bloat in production builds,
> and type names exceeding 127 bytes were silently truncated without
> any indication in debug logs.
>
> Gate the `TYPE_NAME`, `TYPE_NAME_BUF`, and `TYPE_NAME_CSTR` constants
> behind `#[cfg(CONFIG_DEBUG_DEVRES)]` so the 128-byte buffer is only
> allocated when device resource debugging is enabled. For production
> builds, use `crate::c_str!("")` as a zero-cost empty C-string fallback.
>
> When the type name is too long to fit in the 127-byte buffer (plus null
> terminator), the copy routine now appends "..." at the end — copying
> only the first 124 bytes — to clearly indicate truncation rather than
> silently dropping trailing characters.
>
> Signed-off-by: Aary Milind Kinge <kingeaary@gmail.com>
> ---
> rust/kernel/devres.rs | 55 ++++++++++++++++++++++++++++++++++++++++---
> rust/kernel/lib.rs | 1 +
> 2 files changed, 53 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 9e5f93aed20c..79cfe7d9a415 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -185,6 +185,57 @@ pub(super) unsafe fn devres_node_remove(
> }
>
> impl<T: Send> Devres<T> {
> + #[cfg(CONFIG_DEBUG_DEVRES)]
> + const TYPE_NAME: &'static str = core::any::type_name::<T>();
> +
> + #[cfg(CONFIG_DEBUG_DEVRES)]
> + const TYPE_NAME_BUF: [u8; 128] = {
> + let bytes = Self::TYPE_NAME.as_bytes();
> + let mut buf = [0u8; 128];
> + let mut i = 0;
> +
> + if bytes.len() > 127 {
> + // Copy exactly 124 bytes, then append '...' to clearly indicate truncation
> + while i < 124 {
> + buf[i] = bytes[i];
> + i += 1;
> + }
> + buf[124] = b'.';
> + buf[125] = b'.';
> + buf[126] = b'.';
> + buf[127] = 0; // Null terminator
> + } else {
> + // Copy normally
> + while i < bytes.len() {
> + buf[i] = bytes[i];
> + i += 1;
> + }
> + buf[i] = 0; // Null terminator
> + }
> + buf
> + };
> +
> + #[cfg(CONFIG_DEBUG_DEVRES)]
> + const TYPE_NAME_CSTR: &'static crate::str::CStr = {
> + let static_buf: &'static [u8; 128] = &Self::TYPE_NAME_BUF;
> +
> + let mut len = 0;
> + while len < 128 && static_buf[len] != 0 {
> + len += 1;
> + }
> +
> + // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
> + unsafe {
> + crate::str::CStr::from_bytes_with_nul_unchecked(core::slice::from_raw_parts(
> + static_buf.as_ptr(),
> + len + 1,
> + ))
> + }
> + };
> +
> + #[cfg(not(CONFIG_DEBUG_DEVRES))]
> + const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
> +
> /// Creates a new [`Devres`] instance of the given `data`.
> ///
> /// The `data` encapsulated within the returned `Devres` instance' `data` will be
> @@ -209,9 +260,7 @@ pub fn new<E>(dev: &Device<Bound>, data: impl PinInit<T, E>) -> Result<Self>
> unsafe {
> base::devres_set_node_dbginfo(
> node,
> - // TODO: Use `core::any::type_name::<T>()` once it is a `const fn`,
> - // such that we can convert the `&str` to a `&CStr` at compile-time.
> - c"Devres<T>".as_char_ptr(),
> + Self::TYPE_NAME_CSTR.as_char_ptr(),
> core::mem::size_of::<Revocable<T>>(),
> )
> };
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b72b2fbe046d..48df181788f6 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -22,6 +22,7 @@
> // Expected to become stable.
> #![feature(arbitrary_self_types)]
> #![feature(derive_coerce_pointee)]
> +#![feature(const_type_name)]
> //
> // To be determined.
> #![feature(used_with_arg)]
> --
> 2.51.0
>
>
Hi,
This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him
a patch that has triggered this response. He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created. Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.
You are receiving this message because of the following common error(s)
as indicated below:
- This looks like a new version of a previously submitted patch, but you
did not list below the --- line any changes from the previous version.
Please read the section entitled "The canonical patch format" in the
kernel file, Documentation/process/submitting-patches.rst for what
needs to be done here to properly describe this.
If you wish to discuss this problem further, or you have questions about
how to resolve this issue, please feel free to respond to this email and
Greg will reply once he has dug out from the pending patches received
from other developers.
thanks,
greg k-h's patch email bot
On Wed, May 27, 2026 at 4:13 PM Aary Kinge <kingeaary@gmail.com> wrote:
>
>
> Hi Miguel,
>
> On Tue, May 26, 2026 at 6:07 PM Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote:
>>
>> On Tue, May 26, 2026 at 1:58 PM Aary Milind Kinge <kingeaary@gmail.com> wrote:
>> >
>> > The unconditional 128-byte const array allocation for every unique
>> > `Devres<T>` caused unnecessary .rodata bloat in production builds,
>>
>> Wasn't the string deduplicated?
>>
>
> You are right. The original string was identical for all T and would be deduplicated by the linker.
> I will reword the commit message to focus on the actual improvement: replacing the generic placeholder with the real type name
>
>>
>> > terminator), the copy routine now appends "..." at the end — copying
>>
>> Did an LLM assist this patch? If so, please add a tag:
>
>
> Yes. I will add the appropriate tag in v3.
>
>>
>>
>> https://docs.kernel.org/process/generated-content.html
>> https://docs.kernel.org/process/coding-assistants.html
>>
>> Moreover, this should be sent to the right maintainers and reviewers,
>> e.g. at least to "DRIVER CORE, KOBJECTS, DEBUGFS AND SYSFS". Cc'ing
>> them here, but also please Cc all the "RUST" entry.
>
>
> Yes, I will CC the right maintainers.
>
>>
>>
>> > + let mut buf = [0u8; 128];
>>
>> Why is there a hardcoded literal? Please use constants where possible,
>> deriving the rest of the literals from that.
>
>
> I will replace the hardcoded values with derived constants in v3.
>
>>
>>
>> > + let mut len = 0;
>> > + while len < 128 && static_buf[len] != 0 {
>> > + len += 1;
>> > + }
>> > +
>> > + // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
>>
>> This should explain why this is all OK, e.g. why it doesn't go out of
>> bounds (a local-only reading of the loop above would appear to make it
>> so).
>
>
> Understood. I will improve the SAFETY comment and see if the unsafe can
> be avoided entirely.
>
>>
>>
>> Or avoid `unsafe` altogether if possible.
>>
>> > + #[cfg(not(CONFIG_DEBUG_DEVRES))]
>> > + const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
>>
>> Is the name not used at all under `!CONFIG_DEBUG_DEVRES`?
>>
>> Either way, could we move the other `const`s inside this one, and
>> remove the `cfg` here? i.e. reducing the number of conditional items
>> and avoiding to repeat the "signature" of this `const`.
>
>
> Yes, I will restructure this in v3.
>
> I dug into this and it turns out it is used. The naming is misleading — devres_set_node_dbginfo() stores node->name unconditionally, and the trace_devres_log() tracepoint in drivers/base/trace.h records it unconditionally too. The only thing gated behind CONFIG_DEBUG_DEVRES is the devres_dbg() printk, not the tracepoint.
>
> So if I gate the real type name behind CONFIG_DEBUG_DEVRES, the devres tracepoints show an empty string in production traces for every Rust Devres<T> — which is worse than the generic "Devres<T>" placeholder we had before. The correct approach is to just always provide the real type name. The overhead is negligible (a single const array per monomorphization of Devres<T>).
>
> That means I should drop the #[cfg] gating entirely on TYPE_NAME_CSTR and remove the #[cfg(not(CONFIG_DEBUG_DEVRES))] empty-string branch. Thoughts?
>
>>
>>
>> > // Expected to become stable.
>> > #![feature(arbitrary_self_types)]
>> > #![feature(derive_coerce_pointee)]
>> > +#![feature(const_type_name)]
>>
>> New features need to be justified in the commit message, with a link
>> to the tracking issue:
>>
>> https://github.com/rust-lang/rust/issues/63084
>>
>> In particular, please justify why you think it will become stable.
>> Usually, we need to double-check with upstream that is the case.
>
>
> I will update the commit message to justify the feature usage and add
> the tracking issue link, without making claims about stabilization.
>
>>
>>
>> Thanks!
>>
>> Cheers,
>> Miguel
>
>
> Aary
On Tue, May 26, 2026 at 9:50 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Tue, May 26, 2026 at 05:28:25PM +0530, Aary Milind Kinge wrote:
> > The unconditional 128-byte const array allocation for every unique
> > `Devres<T>` caused unnecessary .rodata bloat in production builds,
> > and type names exceeding 127 bytes were silently truncated without
> > any indication in debug logs.
> >
> > Gate the `TYPE_NAME`, `TYPE_NAME_BUF`, and `TYPE_NAME_CSTR` constants
> > behind `#[cfg(CONFIG_DEBUG_DEVRES)]` so the 128-byte buffer is only
> > allocated when device resource debugging is enabled. For production
> > builds, use `crate::c_str!("")` as a zero-cost empty C-string fallback.
> >
> > When the type name is too long to fit in the 127-byte buffer (plus null
> > terminator), the copy routine now appends "..." at the end — copying
> > only the first 124 bytes — to clearly indicate truncation rather than
> > silently dropping trailing characters.
> >
> > Signed-off-by: Aary Milind Kinge <kingeaary@gmail.com>
> > ---
> > rust/kernel/devres.rs | 55 ++++++++++++++++++++++++++++++++++++++++---
> > rust/kernel/lib.rs | 1 +
> > 2 files changed, 53 insertions(+), 3 deletions(-)
> >
> > diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> > index 9e5f93aed20c..79cfe7d9a415 100644
> > --- a/rust/kernel/devres.rs
> > +++ b/rust/kernel/devres.rs
> > @@ -185,6 +185,57 @@ pub(super) unsafe fn devres_node_remove(
> > }
> >
> > impl<T: Send> Devres<T> {
> > + #[cfg(CONFIG_DEBUG_DEVRES)]
> > + const TYPE_NAME: &'static str = core::any::type_name::<T>();
> > +
> > + #[cfg(CONFIG_DEBUG_DEVRES)]
> > + const TYPE_NAME_BUF: [u8; 128] = {
> > + let bytes = Self::TYPE_NAME.as_bytes();
> > + let mut buf = [0u8; 128];
> > + let mut i = 0;
> > +
> > + if bytes.len() > 127 {
> > + // Copy exactly 124 bytes, then append '...' to clearly indicate truncation
> > + while i < 124 {
> > + buf[i] = bytes[i];
> > + i += 1;
> > + }
> > + buf[124] = b'.';
> > + buf[125] = b'.';
> > + buf[126] = b'.';
> > + buf[127] = 0; // Null terminator
> > + } else {
> > + // Copy normally
> > + while i < bytes.len() {
> > + buf[i] = bytes[i];
> > + i += 1;
> > + }
> > + buf[i] = 0; // Null terminator
> > + }
> > + buf
> > + };
> > +
> > + #[cfg(CONFIG_DEBUG_DEVRES)]
> > + const TYPE_NAME_CSTR: &'static crate::str::CStr = {
> > + let static_buf: &'static [u8; 128] = &Self::TYPE_NAME_BUF;
> > +
> > + let mut len = 0;
> > + while len < 128 && static_buf[len] != 0 {
> > + len += 1;
> > + }
> > +
> > + // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
> > + unsafe {
> > + crate::str::CStr::from_bytes_with_nul_unchecked(core::slice::from_raw_parts(
> > + static_buf.as_ptr(),
> > + len + 1,
> > + ))
> > + }
> > + };
> > +
> > + #[cfg(not(CONFIG_DEBUG_DEVRES))]
> > + const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
> > +
> > /// Creates a new [`Devres`] instance of the given `data`.
> > ///
> > /// The `data` encapsulated within the returned `Devres` instance' `data` will be
> > @@ -209,9 +260,7 @@ pub fn new<E>(dev: &Device<Bound>, data: impl PinInit<T, E>) -> Result<Self>
> > unsafe {
> > base::devres_set_node_dbginfo(
> > node,
> > - // TODO: Use `core::any::type_name::<T>()` once it is a `const fn`,
> > - // such that we can convert the `&str` to a `&CStr` at compile-time.
> > - c"Devres<T>".as_char_ptr(),
> > + Self::TYPE_NAME_CSTR.as_char_ptr(),
> > core::mem::size_of::<Revocable<T>>(),
> > )
> > };
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index b72b2fbe046d..48df181788f6 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -22,6 +22,7 @@
> > // Expected to become stable.
> > #![feature(arbitrary_self_types)]
> > #![feature(derive_coerce_pointee)]
> > +#![feature(const_type_name)]
> > //
> > // To be determined.
> > #![feature(used_with_arg)]
> > --
> > 2.51.0
> >
> >
>
> Hi,
>
> This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him
> a patch that has triggered this response. He used to manually respond
> to these common problems, but in order to save his sanity (he kept
> writing the same thing over and over, yet to different people), I was
> created. Hopefully you will not take offence and will fix the problem
> in your patch and resubmit it so that it can be accepted into the Linux
> kernel tree.
>
> You are receiving this message because of the following common error(s)
> as indicated below:
>
> - This looks like a new version of a previously submitted patch, but you
> did not list below the --- line any changes from the previous version.
> Please read the section entitled "The canonical patch format" in the
> kernel file, Documentation/process/submitting-patches.rst for what
> needs to be done here to properly describe this.
>
> If you wish to discuss this problem further, or you have questions about
> how to resolve this issue, please feel free to respond to this email and
> Greg will reply once he has dug out from the pending patches received
> from other developers.
>
> thanks,
>
> greg k-h's patch email bot
On Tue, May 26, 2026 at 1:58 PM Aary Milind Kinge <kingeaary@gmail.com> wrote:
>
> The unconditional 128-byte const array allocation for every unique
> `Devres<T>` caused unnecessary .rodata bloat in production builds,
Wasn't the string deduplicated?
> terminator), the copy routine now appends "..." at the end — copying
Did an LLM assist this patch? If so, please add a tag:
https://docs.kernel.org/process/generated-content.html
https://docs.kernel.org/process/coding-assistants.html
Moreover, this should be sent to the right maintainers and reviewers,
e.g. at least to "DRIVER CORE, KOBJECTS, DEBUGFS AND SYSFS". Cc'ing
them here, but also please Cc all the "RUST" entry.
> + let mut buf = [0u8; 128];
Why is there a hardcoded literal? Please use constants where possible,
deriving the rest of the literals from that.
> + let mut len = 0;
> + while len < 128 && static_buf[len] != 0 {
> + len += 1;
> + }
> +
> + // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
This should explain why this is all OK, e.g. why it doesn't go out of
bounds (a local-only reading of the loop above would appear to make it
so).
Or avoid `unsafe` altogether if possible.
> + #[cfg(not(CONFIG_DEBUG_DEVRES))]
> + const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
Is the name not used at all under `!CONFIG_DEBUG_DEVRES`?
Either way, could we move the other `const`s inside this one, and
remove the `cfg` here? i.e. reducing the number of conditional items
and avoiding to repeat the "signature" of this `const`.
> // Expected to become stable.
> #![feature(arbitrary_self_types)]
> #![feature(derive_coerce_pointee)]
> +#![feature(const_type_name)]
New features need to be justified in the commit message, with a link
to the tracking issue:
https://github.com/rust-lang/rust/issues/63084
In particular, please justify why you think it will become stable.
Usually, we need to double-check with upstream that is the case.
Thanks!
Cheers,
Miguel
On Tue, 26 May 2026 14:37:44 +0200
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote:
> On Tue, May 26, 2026 at 1:58 PM Aary Milind Kinge <kingeaary@gmail.com> wrote:
> >
> > The unconditional 128-byte const array allocation for every unique
> > `Devres<T>` caused unnecessary .rodata bloat in production builds,
>
> Wasn't the string deduplicated?
>
> > terminator), the copy routine now appends "..." at the end — copying
>
> Did an LLM assist this patch? If so, please add a tag:
>
> https://docs.kernel.org/process/generated-content.html
> https://docs.kernel.org/process/coding-assistants.html
>
> Moreover, this should be sent to the right maintainers and reviewers,
> e.g. at least to "DRIVER CORE, KOBJECTS, DEBUGFS AND SYSFS". Cc'ing
> them here, but also please Cc all the "RUST" entry.
>
> > + let mut buf = [0u8; 128];
>
> Why is there a hardcoded literal? Please use constants where possible,
> deriving the rest of the literals from that.
>
> > + let mut len = 0;
> > + while len < 128 && static_buf[len] != 0 {
> > + len += 1;
> > + }
> > +
> > + // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
>
> This should explain why this is all OK, e.g. why it doesn't go out of
> bounds (a local-only reading of the loop above would appear to make it
> so).
I'm no rust expert (or novice) but that code all looks like run-time
initialisers rather that the static data you really want.
Can you generate the '\0' terminated C 'string' by including an explicit
zero byte in the rust one?
-- David
© 2016 - 2026 Red Hat, Inc.