Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

From: David Hildenbrand
Date: Mon Aug 16 2021 - 10:10:40 EST

Until recently, the CPUs only having 4 1GB TLB entries. I'm sure we
still have customers using that generation of CPUs. 2MB pages perform
better than 1GB pages on the previous generation of hardware, and I
haven't seen numbers for the next generation yet.

I read that somewhere else before, yet we have heavy 1 GiB page users,
especially in the context of VMs and DPDK.

I wonder if those users actually benchmarked. Or whether the memory
savings worked out so well for them that the loss of TLB performance
didn't matter.

These applications are extremely performance sensitive (i.e., RT workloads), that's why I'm wondering. I recall that they are most certainly using more than 4 GiB memory in real applications.

E.g., the doc [1] even has a note that "For 64-bit applications, it is recommended to use 1 GB hugepages if the platform supports them."


So, it only works for hugetlbfs in case uffd is not in place (-> no
per-process data in the page table) and we have an actual shared mappings.
When unsharing, we zap the PUD entry, which will result in allocating a
per-process page table on next fault.

I think uffd was a huge mistake. It should have been a filesystem
instead of a hack on the side of anonymous memory.

Yes it was. Especially, looking at all the special-casing, for example, even in mm/pagewalk.c.

I will rephrase my previous statement "hugetlbfs just doesn't raise these
problems because we are special casing it all over the place already". For
example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
hugetlbfs locking.

Sure, that's why I want to drag this feature out of "oh this is a
hugetlb special case" and into "this is something Linux supports".

I would have understood the move to optimize SHMEM internally - similar to how we seem to optimize hugetlbfs SHMEM right now internally. (although sharing page tables for shmem can still be quite tricky)

I did not follow why we have to play games with MAP_PRIVATE, and having private anonymous pages shared between processes that don't COW, introducing new syscalls etc.


David / dhildenb