Re: [PATCH 0/6] hugetlbfs: support free page reporting

[Liang Li]

On Wed, Jan 6, 2021 at 5:41 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 06.01.21 04:46, Liang Li wrote:
> > A typical usage of hugetlbfs it's to reserve amount of memory
> > during the kernel booting stage, and the reserved pages are
> > unlikely to return to the buddy system. When application need
> > hugepages, kernel will allocate them from the reserved pool.
> > when application terminates, huge pages will return to the
> > reserved pool and are kept in the free list for hugetlbfs,
> > these free pages will not return to buddy freelist unless the
> > size of reserved pool is changed.
> > Free page reporting only supports buddy pages, it can't report
> > the free pages reserved for hugetlbfs. On the other hand,
> > hugetlbfs is a good choice for system with a huge amount of RAM,
> > because it can help to reduce the memory management overhead and
> > improve system performance.
> > This patch add the support for reporting hugepages in the free
> > list of hugetlbfs, it can be used by virtio_balloon driver for
> > memory overcommit and pre zero out free pages for speeding up
> > memory population and page fault handling.
>
> You should lay out the use case + measurements. Further you should
> describe what this patch set actually does, how behavior can be tuned,
> pros and cons, etc... And you should most probably keep this RFC.
>
> >
> > Most of the code are 'copied' from free page reporting because
> > they are working in the same way. So the code can be refined to
> > remove duplication. It can be done later.
>
> Nothing speaks about getting it right from the beginning. Otherwise it
> will most likely never happen.
>
> >
> > Since some guys have some concern about side effect of the 'buddy
> > free page pre zero out' feature brings, I remove it from this
> > serier.
>
> You should really point out what changed size the last version. I
> remember Alex and Mike had some pretty solid points of what they don't
> want to see (especially: don't use free page reporting infrastructure
> and don't temporarily allocate huge pages for processing them).
>
> I am not convinced that we want to use the free page reporting
> infrastructure for this (pre-zeroing huge pages). What speaks about a
> thread simply iterating over huge pages one at a time, zeroing them? The
> whole free page reporting infrastructure was invented because we have to
> do expensive coordination (+ locking) when going via the hypervisor. For
> the main use case of zeroing huge pages in the background, I don't see a
> real need for that. If you believe this is the right thing to do, please
> add a discussion regarding this.
>
> --
> Thanks,
>
> David / dhildenb
>
>
I will take all your advice and give more detail in the next revision,
Thanks for your comments!

Liang