linux-kernel - Re: [PATCH v2 1/4] arm64: export memblock

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Date:   Tue, 3 Jul 2018 17:44:37 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, akpm@...ux-foundation.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>, lorenzo.pieralisi@....com,
        graeme.gregory@...aro.org, Al Stone <al.stone@...aro.org>,
        Bhupesh Sharma <bhsharma@...hat.com>, tbaicar@...eaurora.org,
        kexec mailing list <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hanjun Guo <hanjun.guo@...aro.org>, sudeep.holla@....com,
        Dave Young <dyoung@...hat.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/4] arm64: export memblock_reserve()d regions via /proc/iomem

On Tue, Jul 3, 2018 at 12:17 PM, AKASHI Takahiro
<takahiro.akashi@...aro.org> wrote:
> On Tue, Jun 19, 2018 at 10:22:46AM -0500, Dave Kleikamp wrote:
>> On 06/19/2018 10:00 AM, James Morse wrote:
>> > Hi Dave,
>> >
>> > On 19/06/18 14:37, Dave Kleikamp wrote:
>> >> On 06/19/2018 01:44 AM, AKASHI Takahiro wrote:
>> >>> +static int __init reserve_memblock_reserved_regions(void)
>> >>> +{
>> >>> + phys_addr_t start, end, roundup_end = 0;
>> >>> + struct resource *mem, *res;
>> >>> + u64 i;
>> >>> +
>> >>> + for_each_reserved_mem_region(i, &start, &end) {
>> >>> +         if (end <= roundup_end)
>> >>> +                 continue; /* done already */
>> >>> +
>> >>> +         start = __pfn_to_phys(PFN_DOWN(start));
>> >>> +         end = __pfn_to_phys(PFN_UP(end)) - 1;
>> >>> +         roundup_end = end;
>> >>> +
>> >>> +         res = kzalloc(sizeof(*res), GFP_ATOMIC);
>> >>> +         if (WARN_ON(!res))
>> >>> +                 return -ENOMEM;
>> >>> +         res->start = start;
>> >>> +         res->end = end;
>> >>> +         res->name  = "reserved";
>> >>> +         res->flags = IORESOURCE_MEM;
>> >>> +
>> >>> +         mem = request_resource_conflict(&iomem_resource, res);
>> >>> +         /*
>> >>> +          * We expected memblock_reserve() regions to conflict with
>> >>> +          * memory created by request_standard_resources().
>> >>> +          */
>> >>> +         if (WARN_ON_ONCE(!mem))
>> >>> +                 continue;
>> >>> +         kfree(res);
>> >>
>> >> Why is kfree() after the conditional continue? This is a memory leak.
>> >
>> > request_resource_conflict() inserts res into the iomem_resource tree, or returns
>> > the conflict if there is already something at this address.
>> >
>> > We expect something at this address: a 'System RAM' section added by
>> > request_resource(). This code wants the conflicting 'System RAM' entry so that
>> > reserve_region_with_split() can fill in the gaps below it with 'reserved'. See
>> > the commit-message for an example.
>> >
>> > If there was no conflict, it means the memory map doesn't look like we expect,
>> > we can't pass NULL to reserve_region_with_split(), and we just inserted the
>> > 'reserved' at the top level. Freeing res at this point would be a use-after-free
>> > hanging from /proc/iomem.
>> > This code generates a WARN_ON_ONCE() and leaves the 'reserved' description where
>> > it is.
>>
>> Okay. I get it.
>>
>> > Trying to cleanup here is pointless, we can remove the resource entry and free
>> > it ... but we still want to report it as reserved, which is what just happened,
>> > just not quite how we we wanted it.
>> >
>> > If you ever see this warning, it means some RAM stopped being nomap between
>> > request_resources() and reserve_memblock_reserved_regions(). I can't find any
>> > case where that ever happens.
>> >
>> >
>> > If all that makes sense: how can I improve the comment above the WARN_ON_ONCE()
>> > to make it clearer?
>>
>> I guess something like you described above.
>>
>> /*
>>  * We expect a 'System RAM' section at this address. In the unexpected
>>  * case where one is not found, request_resource_conflict() will insert
>>  * res into the iomem_resource tree.
>>  */
>>
>> Do you think this is clearer?
>
> If this is the only change needed in my patchset, I'd like the maintainers
> to take care of it instead of my posting another version.
>

+1.

crashkernel booting on arm64 machines which support boot via acpi
tables is broken since a long time, so it would be great to get these
into upstream asap.

I think the comment can be addressed while picking up the patchset in
the maintainer's tree.

I am not sure whether it will go through the ARM tree (Will) or the
EFI tree (Ard), but since this has a Tested-by (from myself) and
Reviewed-by (from James), probably this can be pulled in to allow
further tests via the maintainer's tree.

Thanks,
Bhupesh


>> >
>> >
>> > Thanks,
>> >
>> > James
>> >
>> >
>> >>> +
>> >>> +         reserve_region_with_split(mem, start, end, "reserved");
>> >>> + }
>> >>> +
>> >>> + return 0;
>> >>> +}
>> >>> +arch_initcall(reserve_memblock_reserved_regions);
>> >>> +
>> >>>  u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };
>> >>>
>> >>>  void __init setup_arch(char **cmdline_p)
>> >>>
>> >