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

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Date:   Wed, 6 Jan 2021 10:41:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 0/6] hugetlbfs: support free page reporting

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