Re: [PATCH] mm/memblock: fix potential issue in memblock_search_pfn_nid()

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Apr 03, 2018 at 01:30:41PM +0200, Michal Hocko wrote:
>On Mon 02-04-18 09:50:26, Wei Yang wrote:
>> On Fri, Mar 30, 2018 at 01:57:27PM -0700, Andrew Morton wrote:
>> >On Fri, 30 Mar 2018 11:30:55 +0800 Wei Yang <richard.weiyang@gmail.com> wrote:
>> >
>> >> memblock_search_pfn_nid() returns the nid and the [start|end]_pfn of the
>> >> memory region where pfn sits in. While the calculation of start_pfn has
>> >> potential issue when the regions base is not page aligned.
>> >> 
>> >> For example, we assume PAGE_SHIFT is 12 and base is 0x1234. Current
>> >> implementation would return 1 while this is not correct.
>> >
>> >Why is this not correct?  The caller might want the pfn of the page
>> >which covers the base?
>> >
>> 
>> Hmm... the only caller of memblock_search_pfn_nid() is __early_pfn_to_nid(),
>> which returns the nid of a pfn and save the [start_pfn, end_pfn] with in the
>> same memory region to a cache. So this looks not a good practice to store
>> un-exact pfn in the cache.
>> 
>> >> This patch fixes this by using PFN_UP().
>> >> 
>> >> The original commit is commit e76b63f80d93 ("memblock, numa: binary search
>> >> node id") and merged in v3.12.
>> >> 
>> >> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> >> Cc: 3.12+ <stable@vger.kernel.org>
>> >
>> >Please fully describe the runtime effects of a bug when fixing that
>> >bug.  This description doesn't give enough justification for merging
>> >the patch into mainline, let alone -stable.
>> 
>> Since PFN_UP() and PFN_DOWN() differs when the address is not page aligned, in
>> theory we may have two situations like below.
>
>Have you ever seen a HW that would report page unaligned memory ranges?
>Is this even possible?

No, so we don't need to handle this case?

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
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