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

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Message-ID: <878rdgvx7v.ffs@tglx>
Date:   Tue, 23 May 2023 00:05:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Song Liu <song@...nel.org>, Mike Rapoport <rppt@...nel.org>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>, 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>,
        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 Fri, May 19 2023 at 08:42, Song Liu wrote:
> On Fri, May 19, 2023 at 1:30 AM Mike Rapoport <rppt@...nel.org> wrote:
>> What I was thinking is that we can replace module_alloc() calls in your
>> allocator with something based on my unmapped_alloc(). If we make the part
>> that refills the cache also take care of creating the mapping in the
>> module address space, that should cover everything.
>
> Here are what I found as I work more on the code:
>
> 1. It takes quite some work to create a clean interface and make sure
>     all the users of module_alloc can use the new allocator on all archs.
>     (archs with text poke need to work with ROX memory from the
>     allocator; archs without text poke need to have a clean fall back
>     mechanism, etc.). Most of this work is independent of the actual
>     allocator, so we can do this part and plug in whatever allocator we
>     want (buddy+slab or vmap-based or any other solutions).
>
> 2. vmap_area based solution will work. And it will be one solution for
>     both < PAGE_SIZE and > PAGE_SIZE allocations. Given
>     module_alloc is not in any hot path (AFAIK), I don't see any
>     practical issues with this solution. It will be a little tricky to place
>     and name the code, as it uses vmalloc logic, but it is technically a
>     module allocator.
>
> I will prioritize building the interface, and migrating users to it. If we
> do this part right, changing the underlying allocator should be
> straightforward.

Correct, that's the only workable approach.

Once you have solved #1 the mechanism of the underlying allocator (#2)
is pretty much exchangeable.

Any attempt to solve #2 before #1 is doomed by definition.

Thanks,

        tglx