kernel: grant: do not pass T::default() #2003
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This changes how grant allocation is structured in grant.rs so that the allocation function does not take a copy of the data to be written when creating a new grant. This data can be large if the grant region is large, and it can cause a stack overflow when the allocate function is called.
| @@ -102,7 +102,7 @@ impl Allocator { | |||
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| // We use `ptr::write` to avoid `Drop`ping the uninitialized memory in | |||
| // case `T` implements the `Drop` trait. | |||
| write(ptr.as_ptr(), data); | |||
| write(ptr.as_ptr(), T::default()); | |||
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I'm pretty uncertain why this change would actually solve the "grants temporarily take up a ton of stack space" problem. T::default() is still going to return a T, which will be its full size and on the stack. Is passing it into the alloc_unowned() function taking double the size of T on the stack or something?
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Per a slack discussion with bradjc, this change greatly increases the possible size of IPC grants before a kernel stack overflow occurs (previously 364 bytes increases to 2364 bytes). I'm guessing some compiler optimizations are making the magic happen here.
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Right, presumably the compiler can better guess to just allocate the space for the default value directly at ptr.as_ptr() as opposed to on the stack first.
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bors r+ |
This changes how grant allocation is structured in grant.rs so that the
allocation function does not take a copy of the data to be written when
creating a new grant. This data can be large if the grant region is
large, and it can cause a stack overflow when the allocate function is
called.
The change is pretty simple, but I'm not sure if there is some subtle reason to not do it this way.
Fixes the IPC stack overflow issue found in #1933. Replaces #1976.
Testing Strategy
Running the hail app on hail.
TODO or Help Wanted
n/a
Documentation Updated
/docs, or no updates are required.Formatting
make prepush.