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memfd: Reduce some syscalls in the on-demand case (#3757)

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* memfd: Reduce some syscalls in the on-demand case

This tweaks the internal organization of the `MemFdSlot` to avoid some
syscalls in the default case as well as opportunistically in the pooling
case. The two cases added here are:

* A `MemFdSlot` is now created with a specified initial size. For
  pooling this is 0 but for the on-demand case this can be non-zero.

* When `instantiate` is called with no prior image and the sizes match
  (as will be the case for on-demand allocation) then `mprotect` is
  skipped entirely.

* In the `clear_and_remain-ready` case the `mprotect` is skipped if the
  heap wasn't grown at all.

This should avoid ever using `mprotect` unnecessarily and makes the
ranges we `mprotect` a bit smaller as well.

* Review comments

* Tweak allow to apply to whole crate
This commit is contained in:
Alex Crichton
2022-02-02 16:09:47 -06:00
committed by GitHub
parent 5deb1f1fbf
commit 8ed79c8f57
5 changed files with 79 additions and 72 deletions
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5
crates/runtime/src/memory.rs
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@@ -158,9 +158,8 @@ impl MmapMemory {
// If a memfd image was specified, try to create the MemFdSlot on top of our mmap.
let memfd = match memfd_image {
Some(image) => {
let base = unsafe { mmap.as_mut_ptr().offset(pre_guard_bytes as isize) };
let len = request_bytes - pre_guard_bytes;
let mut memfd_slot = MemFdSlot::create(base as *mut _, len);
let base = unsafe { mmap.as_mut_ptr().add(pre_guard_bytes) };
let mut memfd_slot = MemFdSlot::create(base.cast(), minimum, alloc_bytes);
memfd_slot.instantiate(minimum, Some(image))?;
// On drop, we will unmap our mmap'd range that this
// memfd_slot was mapped on top of, so there is no