Messages in this thread | | | Date | Tue, 23 May 2017 16:09:47 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages |
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On Mon, May 22, 2017 at 02:35:59PM +0100, Punit Agrawal wrote: > From: Will Deacon <will.deacon@arm.com> > > When operating on hugepages with DEBUG_VM enabled, the GUP code checks the > compound head for each tail page prior to calling page_cache_add_speculative. > This is broken, because on the fast-GUP path (where we don't hold any page > table locks) we can be racing with a concurrent invocation of > split_huge_page_to_list. > > split_huge_page_to_list deals with this race by using page_ref_freeze to > freeze the page and force concurrent GUPs to fail whilst the component > pages are modified. This modification includes clearing the compound_head > field for the tail pages, so checking this prior to a successful call > to page_cache_add_speculative can lead to false positives: In fact, > page_cache_add_speculative *already* has this check once the page refcount > has been successfully updated, so we can simply remove the broken calls > to VM_BUG_ON_PAGE. > > Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> > Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> > Signed-off-by: Will Deacon <will.deacon@arm.com> > Acked-by: Steve Capper <steve.capper@arm.com> > Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Looks reasonable to me:
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
-- Kirill A. Shutemov
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