On Mon, May 22, 2017 at 02:35:59PM +0100, Punit Agrawal wrote:

From: Will Deacon <redacted>

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" <redacted>
Cc: Aneesh Kumar K.V <redacted>
Signed-off-by: Will Deacon <redacted>
Acked-by: Steve Capper <redacted>
Signed-off-by: Punit Agrawal <redacted>

Looks reasonable to me:

Acked-by: Kirill A. Shutemov <redacted>

-- 
 Kirill A. Shutemov