[RFC v4 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

[Yang Shi]

Since tmpfs THP was supported in 4.8, hugetlbfs is not the only
filesystem with huge page support anymore. tmpfs can use huge page via
THP when mounting by "huge=" mount option.

When applications use huge page on hugetlbfs, it just need check the
filesystem magic number, but it is not enough for tmpfs. Make
stat.st_blksize return huge page size if it is mounted by appropriate
"huge=" option to give applications a hint to optimize the behavior with
THP.

Some applications may not do wisely with THP. For example, QEMU may mmap
file on non huge page aligned hint address with MAP_FIXED, which results
in no pages are PMD mapped even though THP is used. Some applications
may mmap file with non huge page aligned offset. Both behaviors make THP
pointless.

statfs.f_bsize still returns 4KB for tmpfs since THP could be split, and it
also may fallback to 4KB page silently if there is not enough huge page.
Furthermore, different f_bsize makes max_blocks and free_blocks
calculation harder but without too much benefit. Returning huge page
size via stat.st_blksize sounds good enough.

Since PUD size huge page for THP has not been supported, now it just
returns HPAGE_PMD_SIZE.

Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
---
v3 --> v4:
* Rework the commit log per the education from Michal and Kirill
* Fix build error if CONFIG_TRANSPARENT_HUGEPAGE is disabled
v2 --> v3:
* Use shmem_sb_info.huge instead of global variable per Michal's comment
v2 --> v1:
* Adopted the suggestion from hch to return huge page size via st_blksize
  instead of creating a new flag.

 mm/shmem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index b859192..19b8055 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -988,6 +988,7 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
 {
 	struct inode *inode = path->dentry->d_inode;
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 
 	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {
 		spin_lock_irq(&info->lock);
@@ -995,6 +996,11 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
 		spin_unlock_irq(&info->lock);
 	}
 	generic_fillattr(inode, stat);
+#ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
+	if (sbinfo->huge > 0)
+		stat->blksize = HPAGE_PMD_SIZE;
+#endif
+	
 	return 0;
 }
 
-- 
1.8.3.1