linux-kernel - Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped

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Message-ID: <ZGZfLHA8vuRJwa0f@moria.home.lan>
Date:   Thu, 18 May 2023 13:23:56 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Song Liu <song@...nel.org>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()

On Thu, May 18, 2023 at 10:00:39AM -0700, Song Liu wrote:
> On Thu, May 18, 2023 at 9:48 AM Kent Overstreet
> <kent.overstreet@...ux.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 planning 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 think
> > he had some fair points in that thread.
> 
> Yes, the API is based on Thomas's suggestion, like 90% from the discussions.
> 
> >
> > My own feeling is that the buddy allocator is our tool for allocating
> > larger variable sized physically contiguous allocations, so I'd like to
> > see something based on that - I think we could do a hybrid buddy/slab
> > 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, so
perhaps I should keep going with my slab allocator.

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 missed
:)