Re: [PATCH v2 00/17] khugepaged: Asynchronous mTHP collapse

[Andrew Morton]

On Tue, 11 Feb 2025 16:43:09 +0530 Dev Jain <dev.jain@xxxxxxx> wrote:

> This patchset extends khugepaged from collapsing only PMD-sized THPs to
> collapsing anonymous mTHPs.
> 
> mTHPs were introduced in the kernel to improve memory management by allocating
> chunks of larger memory, so as to reduce number of page faults, TLB misses (due
> to TLB coalescing), reduce length of LRU lists, etc. However, the mTHP property
> is often lost due to CoW, swap-in/out, and when the kernel just cannot find
> enough physically contiguous memory to allocate on fault. Henceforth, there is a
> need to regain mTHPs in the system asynchronously. This work is an attempt in
> this direction, starting with anonymous folios.
> 
> In the fault handler, we select the THP order in a greedy manner; the same has
> been used here, along with the same sysfs interface to control the order of
> collapse. In contrast to PMD-collapse, we (hopefully) get rid of the mmap_write_lock().
> 
> ---------------------------------------------------------
> Testing
> ---------------------------------------------------------
> 
> The set has been build tested on x86_64.
> For Aarch64,
> 1. mm-selftests: No regressions.
> 2. Analyzing with tools/mm/thpmaps on different userspace programs mapping
>    aligned VMAs of a large size, faulting in basepages/mTHPs (according to sysfs),
>    and then madvise()'ing the VMA, khugepaged is able to 100% collapse the VMAs.

It would be nice to provide some evidence that this patchset actually
makes Linux better for our users, and by how much.

Thanks, I think I'll skip v2 and shall await reviewer input.