Re: powerpc/mm: Differentiate between hugetlb and THP during page walk

[Michael Ellerman <mpe@ellerman.id.au>]

On Fri, 2015-09-10 at 03:02:21 UTC, "Aneesh Kumar K.V" wrote:
> We need to properly identify whether a hugepage is an explicit or
> a transparent hugepage in follow_huge_addr(). We used to depend
> on hugepage shift argument to do that. But in some case that can
> result in wrong results. For ex:
> 
> On finding a transparent hugepage we set hugepage shift to PMD_SHIFT.
> But we can end up clearing the thp pte, via pmdp_huge_get_and_clear.
> We do prevent reusing the pfn page via the usage of
> kick_all_cpus_sync(). But that happens after we updated the pte to 0.
> Hence in follow_huge_addr() we can find hugepage shift set, but transparent
> huge page check fail for a thp pte.
> 
> NOTE: We fixed a variant of this race against thp split in commit
> 691e95fd7396905a38d98919e9c150dbc3ea21a3
> ("powerpc/mm/thp: Make page table walk safe against thp split/collapse")
> 
> Without this patch, we may hit the BUG_ON(flags & FOLL_GET) in
> follow_page_mask occasionally.
> 
> In the long term, we may want to switch ppc64 64k page size config to
> enable CONFIG_ARCH_WANT_GENERAL_HUGETLB
> 
> Reported-by: David Gibson <david@gibson.dropbear.id.au>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/891121e6c02c6242487aa4ea

cheers