[PATCHv6 14/22] thp, mm: allocate huge pages in grab_cache_page_write_begin()

[Kirill A. Shutemov]

Try to allocate huge page if flags has AOP_FLAG_TRANSHUGE.

If, for some reason, it's not possible allocate a huge page at this
possition, it returns NULL. Caller should take care of fallback to
small pages.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 include/linux/fs.h |  1 +
 mm/filemap.c       | 23 +++++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 26801f0bb1..42ccdeddd9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -282,6 +282,7 @@ enum positive_aop_returns {
 #define AOP_FLAG_NOFS			0x0004 /* used by filesystem to direct
 						* helper code (eg buffer layer)
 						* to clear GFP_FS from alloc */
+#define AOP_FLAG_TRANSHUGE		0x0008 /* allocate transhuge page */
 
 /*
  * oh the beauties of C type declarations.
diff --git a/mm/filemap.c b/mm/filemap.c
index 3421bcaed4..410879a801 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2322,18 +2322,37 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
 	gfp_t gfp_mask;
 	struct page *page;
 	gfp_t gfp_notmask = 0;
+	bool must_use_thp = (flags & AOP_FLAG_TRANSHUGE) &&
+		IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE);
 
 	gfp_mask = mapping_gfp_mask(mapping);
+	if (must_use_thp) {
+		BUG_ON(index & HPAGE_CACHE_INDEX_MASK);
+		BUG_ON(!(gfp_mask & __GFP_COMP));
+	}
 	if (mapping_cap_account_dirty(mapping))
 		gfp_mask |= __GFP_WRITE;
 	if (flags & AOP_FLAG_NOFS)
 		gfp_notmask = __GFP_FS;
 repeat:
 	page = find_lock_page(mapping, index);
-	if (page)
+	if (page) {
+		if (must_use_thp && !PageTransHuge(page)) {
+			unlock_page(page);
+			page_cache_release(page);
+			return NULL;
+		}
 		goto found;
+	}
 
-	page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
+	if (must_use_thp) {
+		page = alloc_pages(gfp_mask & ~gfp_notmask, HPAGE_PMD_ORDER);
+		if (page)
+			count_vm_event(THP_WRITE_ALLOC);
+		else
+			count_vm_event(THP_WRITE_ALLOC_FAILED);
+	} else
+		page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
 	if (!page)
 		return NULL;
 	status = add_to_page_cache_lru(page, mapping, index,
-- 
1.8.4.rc3

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