linux-kernel - Re: [PATCH] riscv: reserve DTB before possible memblock allocation

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Message-ID: <CAHVXubgL8TZydzWogqxNczNkKin=qLp4i6=5d0YVBQdqMtFVjw@mail.gmail.com>
Date:   Mon, 12 Jun 2023 08:57:52 +0200
From:   Alexandre Ghiti <alexghiti@...osinc.com>
To:     Woody Zhang <woodylab@...mail.com>
Cc:     Conor Dooley <conor@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: reserve DTB before possible memblock allocation

On Sun, Jun 11, 2023 at 1:27 AM Woody Zhang <woodylab@...mail.com> wrote:
>
> On Sat, Jun 10, 2023 at 04:41:17PM +0100, Conor Dooley wrote:
> >On Thu, Jun 08, 2023 at 09:49:44AM +0200, Alexandre Ghiti wrote:
> >> On Wed, Jun 7, 2023 at 8:17 PM Conor Dooley <conor@...nel.org> wrote:
> >> >
> >> > +CC Alex, you should take a look at this patch.
> >> >
> >> > On Wed, Jun 07, 2023 at 09:35:19PM +0800, Woody Zhang wrote:
> >> > > It's possible that early_init_fdt_scan_reserved_mem() allocates memory
> >> > > from memblock for dynamic reserved memory in `/reserved-memory` node.
> >> > > Any fixed reservation must be done before that to avoid potential
> >> > > conflicts.
> >> > >
> >> > > Reserve the DTB in memblock just after early scanning it.
> >> >
> >> > The rationale makes sense to me, I am just wondering what compelling
> >> > reason there is to move it away from the memblock_reserve()s for the
> >> > initd and vmlinux? Moving it above early_init_fdt_scan_reserved_mem()
> >> > should be the sufficient minimum & would keep things together.
> >
> >> Thanks Conor.
> >>
> >> So the patch looks good to me.
> >>
> >> But I find this fragile:
> >>
> >> - we do not check memblock_reserve() return value to make sure the
> >> reservation really happened (and quickly looking at the code, I'm not
> >> even sure it returns an error if the region was already allocated).
> >> - we have to make sure no memblock allocation happens before setup_bootmem().
> >> - we also have to check that no fixed memblock_reserve() happens after.
> >>
> >> The last 2 points may sound natural, but we'll have to take great care
> >> when adding some code around here. I'm working on an "early boot
> >> document" and I'll add something about that, but a runtime thing would
> >> be way better IMO.
> >>
> >> You can add:
> >>
> >> Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>
> >
> >btw Alex/Woody, what is the appropriate Fixes tag here?
>
> In ef69d2559fe9 ("riscv: Move early dtb mapping into the fixmap region"),
> Alex move early_init_fdt_scan_reserved_mem to setup_bootmem() to prevent
> memory allocations before of reservations. But it should not be put before
> DTB reservation.
>

Yep, that's the culprit, let's add the proper tag:

Fixes: ef69d2559fe9 ("riscv: Move early dtb mapping into the fixmap region")

Thanks!

Alex

>
> Woody
>