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From:   Song Liu <song@kernel.org>
Date:   Thu, 18 May 2023 12:03:03 -0700
Message-ID: <CAPhsuW4Mm8z4kbVo8-sPU=QL2B1Sb32ZO7teWT8qienGNuxaeQ@mail.gmail.com>
Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     Mike Rapoport <rppt@kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        x86@kernel.org
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On Thu, May 18, 2023 at 11:47=E2=80=AFAM Song Liu <song@kernel.org> wrote:
>
> On Thu, May 18, 2023 at 10:24=E2=80=AFAM Kent Overstreet
> <kent.overstreet@linux.dev> wrote:
> >
> > On Thu, May 18, 2023 at 10:00:39AM -0700, Song Liu wrote:
> > > On Thu, May 18, 2023 at 9:48=E2=80=AFAM Kent Overstreet
> > > <kent.overstreet@linux.dev> wrote:
> > > >
> > > > On Thu, May 18, 2023 at 09:33:20AM -0700, Song Liu wrote:
> > > > > I am working on patches based on the discussion in [1]. I am plan=
ning to
> > > > > send v1 for review in a week or so.
> > > >
> > > > Hey Song, I was reviewing that thread too,
> > > >
> > > > Are you taking a different approach based on Thomas's feedback? I t=
hink
> > > > he had some fair points in that thread.
> > >
> > > Yes, the API is based on Thomas's suggestion, like 90% from the discu=
ssions.
> > >
> > > >
> > > > My own feeling is that the buddy allocator is our tool for allocati=
ng
> > > > larger variable sized physically contiguous allocations, so I'd lik=
e to
> > > > see something based on that - I think we could do a hybrid buddy/sl=
ab
> > > > allocator approach, like we have for regular memory allocations.
> > >
> > > I am planning to implement the allocator based on this (reuse
> > > vmap_area logic):
> >
> > Ah, you're still doing vmap_area approach.
> >
> > Mike's approach looks like it'll be _much_ lighter weight and higher
> > performance, to me. vmalloc is known to be slow compared to the buddy
> > allocator, and with Mike's approach we're only modifying mappings once
> > per 2 MB chunk.
> >
> > I don't see anything in your code for sub-page sized allocations too, s=
o
> > perhaps I should keep going with my slab allocator.
>
> The vmap_area approach handles sub-page allocations. In 5/5 of set [2],
> we showed that multiple BPF programs share the same page with some
> kernel text (_etext).
>
> > Could you share your thoughts on your approach vs. Mike's? I'm newer to
> > this area of the code than you two so maybe there's an angle I've misse=
d
> > :)
>
> AFAICT, tree based solution (vmap_area) is more efficient than bitmap
> based solution.
>
> First, for 2MiB page with 64B chunk size, we need a bitmap of
>      2MiB / 64B =3D 32k bit =3D 4k bytes
> While the tree based solution can adapt to the number of allocations with=
in
> This 2MiB page. Also, searching a free range within 4kB of bitmap may
> actually be slower than searching in the tree.
>
> Second, bitmap based solution cannot handle > 2MiB allocation cleanly,
> while tree based solution can. For example, if a big driver uses 3MiB, th=
e
> tree based allocator can allocate 4MiB for it, and use the rest 1MiB for
> smaller allocations.

 Missed one:

Third, bitmap based solution requires a "size" parameter in free(). It is a=
n
overhead for the user. Tree based solution doesn't have this issue.

Thanks,
Song

>
> [2] https://lore.kernel.org/linux-mm/20221107223921.3451913-6-song@kernel=
.org/