linux-kernel - Re: collision between ZONE_MOVABLE and memblock allocations

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Message-ID: <20230721112009.GP1901145@kernel.org>
Date:   Fri, 21 Jul 2023 14:20:09 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Ross Zwisler <zwisler@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>
Subject: Re: collision between ZONE_MOVABLE and memblock allocations

On Wed, Jul 19, 2023 at 04:26:04PM -0600, Ross Zwisler wrote:
> On Wed, Jul 19, 2023 at 08:44:34AM +0300, Mike Rapoport wrote:
> > 3. Switch memblock to use bottom up allocations. Historically memblock
> > allocated memory from the top to avoid corrupting the kernel image and to
> > avoid exhausting precious ZONE_DMA. I believe we can use bottom-up
> > allocations with lower limit of memblock allocations set to 16M.
> > 
> > With the hack below no memblock allocations will end up in ZONE_MOVABLE:
> 
> Yep, I've confirmed that for my use cases at least this does the trick, thank
> you!  I had thought about moving the memblock allocations, but had no idea it
> was (basically) already supported and thought it'd be much riskier than just
> adjusting where ZONE_MOVABLE lived.
> 
> Is there a reason for this to not be a real option for users, maybe per a
> kernel config knob or something?  I'm happy to explore other options in this
> thread, but this is doing the trick so far.

I think we can make x86 always use bottom up.

To do this properly we'd need to set lower limit for memblock allocations
to MAX_DMA32_PFN and allow fallback below it so that early allocations
won't eat memory from ZONE_DMA32.

Aside from x86 boot being fragile in general I don't see why this wouldn't
work.

-- 
Sincerely yours,
Mike.