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

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Message-Id: <20250211152341.3431089327c5e0ec6ba6064d@linux-foundation.org>
Date: Tue, 11 Feb 2025 15:23:41 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Dev Jain <dev.jain@....com>
Cc: david@...hat.com, willy@...radead.org, kirill.shutemov@...ux.intel.com,
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Subject: Re: [PATCH v2 00/17] khugepaged: Asynchronous mTHP collapse

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