Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  7 Jan 2026 19:31:22 +0800 "Li Zhe" <lizhe.67@bytedance.com> wrote:

> This patchset is based on this commit[1]("mm/hugetlb: optionally
> pre-zero hugetlb pages").
> 
> Fresh hugetlb pages are zeroed out when they are faulted in,
> just like with all other page types. This can take up a good
> amount of time for larger page sizes (e.g. around 250
> milliseconds for a 1G page on a Skylake machine).
> 
> This normally isn't a problem, since hugetlb pages are typically
> mapped by the application for a long time, and the initial
> delay when touching them isn't much of an issue.
> 
> However, there are some use cases where a large number of hugetlb
> pages are touched when an application starts (such as a VM backed
> by these pages), rendering the launch noticeably slow.
> 
> On an Skylake platform running v6.19-rc2, faulting in 64 × 1 GB huge
> pages takes about 16 seconds, roughly 250 ms per page. Even with
> Ankur’s optimizations[2], the time drops only to ~13 seconds,
> ~200 ms per page, still a noticeable delay.
> 
> To accelerate the above scenario, this patchset exports a per-node,
> read-write "zeroable_hugepages" sysfs interface for every hugepage size.
> This interface reports how many hugepages on that node can currently
> be pre-zeroed and allows user space to request that any integer number
> in the range [0, max] be zeroed in a single operation.
> 
> This mechanism offers the following advantages:
> 
> (1) User space gains full control over when zeroing is triggered,
> enabling it to minimize the impact on both CPU and cache utilization.
> 
> (2) Applications can spawn as many zeroing processes as they need,
> enabling concurrent background zeroing.
> 
> (3) By binding the process to specific CPUs, users can confine zeroing
> threads to cores that do not run latency-critical tasks, eliminating
> interference.
> 
> (4) A zeroing process can be interrupted at any time through standard
> signal mechanisms, allowing immediate cancellation.
> 
> (5) The CPU consumption incurred by zeroing can be throttled and contained
> with cgroups, ensuring that the cost is not borne system-wide.
> 
> Tested on the same Skylake platform as above, when the 64 GiB of memory
> was pre-zeroed in advance by the pre-zeroing mechanism, the faulting
> latency test completed in negligible time.
> 
> In user space, we can use system calls such as epoll and write to zero
> huge folios as they become available, and sleep when none are ready. The
> following pseudocode illustrates this approach. The pseudocode spawns
> eight threads (each running thread_fun()) that wait for huge pages on
> node 0 to become eligible for zeroing; whenever such pages are available,
> the threads clear them in parallel.

This seems to be quite a lot of messing around in userspace.  Perhaps
unavoidable given the tradeoffs which are involved, and reasonable in
the sort of environments in which this will be used.  I guess there are
many alternatives - let's see what others think.

>  fs/hugetlbfs/inode.c    |   3 +-
>  include/linux/hugetlb.h |  26 +++++
>  mm/hugetlb.c            | 131 ++++++++++++++++++++++---
>  mm/hugetlb_internal.h   |   6 ++
>  mm/hugetlb_sysfs.c      | 206 ++++++++++++++++++++++++++++++++++++----
>  5 files changed, 337 insertions(+), 35 deletions(-)

Let's find places in Documentation/ (and Documentation/ABI) to document
the userspace interface?