linux-kernel - Re: [PATCH v35 1/5] mm: support to get hints of free page blocks

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Message-ID: <CA+55aFwku2tDH4+rfaC67xc4-cEwSrXgnQaci=e2id5ZCRE9JQ@mail.gmail.com>
Date:   Wed, 11 Jul 2018 09:23:54 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     wei.w.wang@...el.com, virtio-dev@...ts.oasis-open.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        KVM list <kvm@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu0@...il.com, nilal@...hat.com,
        Rik van Riel <riel@...hat.com>, peterx@...hat.com
Subject: Re: [PATCH v35 1/5] mm: support to get hints of free page blocks

On Wed, Jul 11, 2018 at 2:21 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> We already have an interface for that. alloc_pages(GFP_NOWAIT, MAX_ORDER -1).
> So why do we need any array based interface?

That was actually my original argument in the original thread - that
the only new interface people might want is one that just tells how
many of those MAX_ORDER-1 pages there are.

See the thread in v33 with the subject

  "[PATCH v33 1/4] mm: add a function to get free page blocks"

and look for me suggesting just using

    #define GFP_MINFLAGS (__GFP_NORETRY | __GFP_NOWARN |
__GFP_THISNODE | __GFP_NOMEMALLOC)

    struct page *page =  alloc_pages(GFP_MINFLAGS, MAX_ORDER-1);

for this all.

But I could also see an argument for "allocate N pages of size
MAX_ORDER-1", with some small N, simply because I can see the
advantage of not taking and releasing the locking and looking up the
zone individually N times.

If you want to get gigabytes of memory (or terabytes), doing it in
bigger chunks than one single maximum-sized page sounds fairly
reasonable.

I just don't think that "thousands of pages" is reasonable. But "tens
of max-sized pages" sounds fair enough to me, and it would certainly
not be a pain for the VM.

So I'm open to new interfaces. I just want those new interfaces to
make sense, and be low latency and simple for the VM to do. I'm
objecting to the incredibly baroque and heavy-weight one that can
return near-infinite amounts of memory.

The real advantage of jjuist the existing "alloc_pages()" model is
that I think the ballooning people can use that to *test* things out.
If it turns out that taking and releasing the VM locks is a big cost,
we can see if a batch interface that allows you to get tens of pages
at the same time is worth it.

So yes, I'd suggest starting with just the existing alloc_pages. Maybe
it's not enough, but it should be good enough for testing.

                    Linus