[PATCH v4 3/8] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages

[Punit Agrawal]

From: Will Deacon <will.deacon@xxxxxxx>

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.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Steve Capper <steve.capper@xxxxxxx>
Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
Acked-by: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
---
 mm/gup.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index d9e6fddcc51f..ccf8cb38234f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1361,7 +1361,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	head = pmd_page(orig);
 	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
@@ -1400,7 +1399,6 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	head = pud_page(orig);
 	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
@@ -1438,7 +1436,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 	head = pgd_page(orig);
 	page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
-- 
2.11.0