[PATCH] D129107: [BOLT][HUGIFY] adds huge pages support of PIE/no-PIE binaries

[Rafael Auler via Phabricator via llvm-commits]

rafauler added a comment.

Thanks for explaining, Alexey.

Here's what I understood:  from the left side, if we take the last allocated address according to PT_LOAD segments, and then try to put a new segment right after it, aligned to 2MB, old loader somehow ignores p_align and loads that into a 4KB address. But if we give 2MB more space, it does align. This seems quite arbitrary, do you confirm this is what you're seeing and that this is the right fix? If yes, this is odd enough that I think it makes sense for us to put this under a special option -hugify=oldkernel.

>From the right side, we create multiple ELF sections in the same segment and that crashes the loader. The loader would like to see a single segment aligned at 2MB. Is that it? This also would be better under a special -hugify=oldkernel option.

And then restore the behavior in the hugify library to copy all code instead of just hot code, otherwise we're going to crash if -hugify is used instead of -hugify=oldkernel.

When you guard the code under -hugify=oldkernel, please add a comment linking to this diff for the discussion, so somebody reading the code has the background to understand why this was added in the first place.  Weird behaviors such as this need to be thoroughly documented, so it is clear we're compensating for a bug in the system.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D129107/new/

https://reviews.llvm.org/D129107