linux-kernel - [RFC v5 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

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Date:   Wed, 25 Apr 2018 22:13:53 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        mhocko@...nel.org, hch@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org
Cc:     yang.shi@...ux.alibaba.com, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [RFC v5 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

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@...ux.alibaba.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Suggested-by: Christoph Hellwig <hch@...radead.org>
---
v4 --> v5:
* Adopted suggestion from Kirill to use IS_ENABLED and check 'force' and
  'deny'. Extracted the condition into an inline helper.
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 | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index b859192..e9e888b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -571,6 +571,16 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 }
 #endif /* CONFIG_TRANSPARENT_HUGE_PAGECACHE */

+static inline bool is_huge_enabled(struct shmem_sb_info *sbinfo)
+{
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
+	    (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge) &&
+	    shmem_huge != SHMEM_HUGE_DENY)
+		return true;
+	else
+		return false;
+}
+
 /*
  * Like add_to_page_cache_locked, but error if expected item has gone.
  */
@@ -988,6 +998,7 @@ 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 *sb_info = SHMEM_SB(inode->i_sb);

 	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {
 		spin_lock_irq(&info->lock);
@@ -995,6 +1006,10 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
 		spin_unlock_irq(&info->lock);
 	}
 	generic_fillattr(inode, stat);
+
+	if (is_huge_enabled(sb_info))
+		stat->blksize = HPAGE_PMD_SIZE;
+
 	return 0;
 }

-- 
1.8.3.1