[PATCH uprobe, thp v2 3/5] mm, thp: introduce FOLL_SPLIT_PMD

[Song Liu]

This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name says
FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge
page stays as-is.

FOLL_SPLIT_PMD is useful for cases where we need to use regular pages,
but would switch back to huge page and huge pmd on. One of such example
is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe.

Signed-off-by: Song Liu <songliubraving@xxxxxx>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 15 ++++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1bdaf1872492..8b5f4a9aea0b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2633,6 +2633,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_COW	0x4000	/* internal GUP flag */
 #define FOLL_ANON	0x8000	/* don't do file mappings */
 #define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
+#define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
 
 /*
  * NOTE on FOLL_LONGTERM:
diff --git a/mm/gup.c b/mm/gup.c
index 63ac50e48072..bdc350d95d99 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -398,7 +398,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 		spin_unlock(ptl);
 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 	}
-	if (flags & FOLL_SPLIT) {
+	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
 		int ret;
 		page = pmd_page(*pmd);
 		if (is_huge_zero_page(page)) {
@@ -407,7 +407,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 			split_huge_pmd(vma, pmd, address);
 			if (pmd_trans_unstable(pmd))
 				ret = -EBUSY;
-		} else {
+		} else if (flags & FOLL_SPLIT) {
 			if (unlikely(!try_get_page(page))) {
 				spin_unlock(ptl);
 				return ERR_PTR(-ENOMEM);
@@ -419,8 +419,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 			put_page(page);
 			if (pmd_none(*pmd))
 				return no_page_table(vma, flags);
-		}
+		} else {  /* flags & FOLL_SPLIT_PMD */
+			pte_t *pte;
 
+			spin_unlock(ptl);
+			split_huge_pmd(vma, pmd, address);
+			pte = get_locked_pte(mm, address, &ptl);
+			if (!pte)
+				return no_page_table(vma, flags);
+			spin_unlock(ptl);
+			ret = 0;
+		}
 		return ret ? ERR_PTR(ret) :
 			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 	}
-- 
2.17.1