Re: [PATCH v5 1/9] MIPS: Avoid future duplicate elf core header reservation

[Mike Rapoport <rppt@kernel.org>]

On Mon, Aug 23, 2021 at 09:44:55AM -0500, Rob Herring wrote:
> On Mon, Aug 23, 2021 at 8:10 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > On Mon, Aug 23, 2021 at 12:17:50PM +0200, Geert Uytterhoeven wrote:
> > > Hi Mike,
> > >
> > > On Mon, Aug 16, 2021 at 7:52 AM Mike Rapoport <rppt@kernel.org> wrote:
> > > > On Wed, Aug 11, 2021 at 10:50:59AM +0200, Geert Uytterhoeven wrote:
> > > > > Prepare for early_init_fdt_scan_reserved_mem() reserving the memory
> > > > > occupied by an elf core header described in the device tree.
> > > > > As arch_mem_init() calls early_init_fdt_scan_reserved_mem() before
> > > > > mips_reserve_vmcore(), the latter needs to check if the memory has
> > > > > already been reserved before.
> > > >
> > > > Doing memblock_reserve() for the same region is usually fine, did you
> > > > encounter any issues without this patch?
> > >
> > > Does it also work if the same region is part of an earlier larger
> > > reservation?  I am no memblock expert, so I don't know.
> > > I didn't run into any issues, as my MIPS platform is non-DT, but I
> > > assume arch/arm64/mm/init.c:reserve_elfcorehdr() had the check for
> > > a reason.
> >
> > The memory will be reserved regardless of the earlier reservation, the
> > issue may appear when the reservations are made for different purpose. E.g.
> > if there was crash kernel allocation before the reservation of elfcorehdr.
> >
> > The check in such case will prevent the second reservation, but, at least
> > in arch/arm64/mm/init.c:reserve_elfcorehdr() it does not seem to prevent
> > different users of the overlapping regions to step on each others toes.
> 
> If the kernel has been passed in overlapping regions, is there
> anything you can do other than hope to get a message out?

Nothing really. I've been thinking about adding flags to memblock.reserved
to at least distinguish firmware regions from the kernel allocations, but I
never got to that.
 
> > Moreover, arm64::reserve_elfcorehdr() seems buggy to me, because of there
> > is only a partial overlap of the elfcorehdr with the previous reservation,
> > the non-overlapping part of elfcorehdr won't get reserved at all.
> 
> What do you suggest as the arm64 version is not the common version?

I'm not really familiar with crash dump internals, so I don't know if
resetting elfcorehdr_addr to ELFCORE_ADDR_ERR is a good idea. I think at
least arm64::reserve_elfcorehdr() should reserve the entire elfcorehdr area
regardless of the overlap. Otherwise it might get overwritten by a random
memblock_alloc().

> Rob

-- 
Sincerely yours,
Mike.