mm: gup: remove FOLL_SPLIT

Since commit 5a52c9d ("uprobe: use FOLL_SPLIT_PMD instead of
FOLL_SPLIT") and commit ba925fa ("s390/gmap: improve THP splitting")
FOLL_SPLIT has not been used anymore.  Remove the dead code.

Link: https://lkml.kernel.org/r/20210330203900.9222-1-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/transhuge.rst

@@ -53,11 +53,6 @@ prevent the page from being split by anyone.
    of handling GUP on hugetlbfs will also work fine on transparent
    hugepage backed mappings.
 
-In case you can't handle compound pages if they're returned by
-follow_page, the FOLL_SPLIT bit can be specified as a parameter to
-follow_page, so that it will split the hugepages before returning
-them.
-
 Graceful fallback
 =================
 

include/linux/mm.h

@@ -2791,7 +2791,6 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
 				 * and return without waiting upon it */
 #define FOLL_POPULATE	0x40	/* fault in page */
-#define FOLL_SPLIT	0x80	/* don't return transhuge pages, split them */
 #define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
 #define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
 #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */

mm/gup.c

@@ -516,18 +516,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		}
 	}
 
-	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
-		get_page(page);
-		pte_unmap_unlock(ptep, ptl);
-		lock_page(page);
-		ret = split_huge_page(page);
-		unlock_page(page);
-		put_page(page);
-		if (ret)
-			return ERR_PTR(ret);
-		goto retry;
-	}
-
 	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
 	if (unlikely(!try_grab_page(page, flags))) {
 		page = ERR_PTR(-ENOMEM);
@@ -672,7 +660,7 @@ 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 | FOLL_SPLIT_PMD)) {
+	if (flags & FOLL_SPLIT_PMD) {
 		int ret;
 		page = pmd_page(*pmd);
 		if (is_huge_zero_page(page)) {
@@ -681,19 +669,7 @@ 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 if (flags & FOLL_SPLIT) {
-			if (unlikely(!try_get_page(page))) {
-				spin_unlock(ptl);
-				return ERR_PTR(-ENOMEM);
-			}
-			spin_unlock(ptl);
-			lock_page(page);
-			ret = split_huge_page(page);
-			unlock_page(page);
-			put_page(page);
-			if (pmd_none(*pmd))
-				return no_page_table(vma, flags);
-		} else {  /* flags & FOLL_SPLIT_PMD */
+		} else {
 			spin_unlock(ptl);
 			split_huge_pmd(vma, pmd, address);
 			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;