[PATCH v10 7/7] mm,thp: avoid writes to file with THP in pagecache

[Song Liu <songliubraving@fb.com>]

In previous patch, an application could put part of its text section in
THP via madvise(). These THPs will be protected from writes when the
application is still running (TXTBSY). However, after the application
exits, the file is available for writes.

This patch avoids writes to file THP by dropping page cache for the file
when the file is open for write. A new counter nr_thps is added to struct
address_space. In do_dentry_open(), if the file is open for write and
nr_thps is non-zero, we drop page cache for the whole file.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Rik van Riel <riel@surriel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 fs/inode.c         |  3 +++
 fs/open.c          |  8 ++++++++
 include/linux/fs.h | 32 ++++++++++++++++++++++++++++++++
 mm/filemap.c       |  1 +
 mm/khugepaged.c    |  4 +++-
 5 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 0f1e3b563c47..ef33fdf0105f 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -181,6 +181,9 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	mapping->flags = 0;
 	mapping->wb_err = 0;
 	atomic_set(&mapping->i_mmap_writable, 0);
+#ifdef CONFIG_READ_ONLY_THP_FOR_FS
+	atomic_set(&mapping->nr_thps, 0);
+#endif
 	mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);
 	mapping->private_data = NULL;
 	mapping->writeback_index = 0;
diff --git a/fs/open.c b/fs/open.c
index a59abe3c669a..c60cd22cc052 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -818,6 +818,14 @@ static int do_dentry_open(struct file *f,
 		if (!f->f_mapping->a_ops || !f->f_mapping->a_ops->direct_IO)
 			return -EINVAL;
 	}
+
+	/*
+	 * XXX: Huge page cache doesn't support writing yet. Drop all page
+	 * cache for this file before processing writes.
+	 */
+	if ((f->f_mode & FMODE_WRITE) && filemap_nr_thps(inode->i_mapping))
+		truncate_pagecache(inode, 0);
+
 	return 0;
 
 cleanup_all:
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 56b8e358af5c..29b2729bd48a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -427,6 +427,7 @@ int pagecache_write_end(struct file *, struct address_space *mapping,
  * @i_pages: Cached pages.
  * @gfp_mask: Memory allocation flags to use for allocating pages.
  * @i_mmap_writable: Number of VM_SHARED mappings.
+ * @nr_thps: Number of THPs in the pagecache (non-shmem only).
  * @i_mmap: Tree of private and shared mappings.
  * @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable.
  * @nrpages: Number of page entries, protected by the i_pages lock.
@@ -444,6 +445,10 @@ struct address_space {
 	struct xarray		i_pages;
 	gfp_t			gfp_mask;
 	atomic_t		i_mmap_writable;
+#ifdef CONFIG_READ_ONLY_THP_FOR_FS
+	/* number of thp, only for non-shmem files */
+	atomic_t		nr_thps;
+#endif
 	struct rb_root_cached	i_mmap;
 	struct rw_semaphore	i_mmap_rwsem;
 	unsigned long		nrpages;
@@ -2775,6 +2780,33 @@ static inline errseq_t filemap_sample_wb_err(struct address_space *mapping)
 	return errseq_sample(&mapping->wb_err);
 }
 
+static inline int filemap_nr_thps(struct address_space *mapping)
+{
+#ifdef CONFIG_READ_ONLY_THP_FOR_FS
+	return atomic_read(&mapping->nr_thps);
+#else
+	return 0;
+#endif
+}
+
+static inline void filemap_nr_thps_inc(struct address_space *mapping)
+{
+#ifdef CONFIG_READ_ONLY_THP_FOR_FS
+	atomic_inc(&mapping->nr_thps);
+#else
+	WARN_ON_ONCE(1);
+#endif
+}
+
+static inline void filemap_nr_thps_dec(struct address_space *mapping)
+{
+#ifdef CONFIG_READ_ONLY_THP_FOR_FS
+	atomic_dec(&mapping->nr_thps);
+#else
+	WARN_ON_ONCE(1);
+#endif
+}
+
 extern int vfs_fsync_range(struct file *file, loff_t start, loff_t end,
 			   int datasync);
 extern int vfs_fsync(struct file *file, int datasync);
diff --git a/mm/filemap.c b/mm/filemap.c
index 0b8b117dbed6..45940d1f59db 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -205,6 +205,7 @@ static void unaccount_page_cache_page(struct address_space *mapping,
 			__dec_node_page_state(page, NR_SHMEM_THPS);
 	} else if (PageTransHuge(page)) {
 		__dec_node_page_state(page, NR_FILE_THPS);
+		filemap_nr_thps_dec(mapping);
 	}
 
 	/*
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8fb26856a7e9..80735bf2a70e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1653,8 +1653,10 @@ static void collapse_file(struct mm_struct *mm,
 
 	if (is_shmem)
 		__inc_node_page_state(new_page, NR_SHMEM_THPS);
-	else
+	else {
 		__inc_node_page_state(new_page, NR_FILE_THPS);
+		filemap_nr_thps_inc(mapping);
+	}
 
 	if (nr_none) {
 		struct zone *zone = page_zone(new_page);
-- 
2.17.1