[PATCH -next] ovl: respect underlying filesystem's get_unmapped_area()

[Jinjiang Tu <tujinjiang@huawei.com>]

During our tests in containers, there is a read-only file (i.e., shared
libraies) in the overlayfs filesystem, and the underlying filesystem is
ext4, which supports large folio. We mmap the file with PROT_READ prot,
and then call madvise(MADV_COLLAPSE) for it. However, the madvise call
fails and returns EINVAL.

The reason is that the mapping address isn't aligned to PMD size. Since
overlayfs doesn't support large folio, __get_unmapped_area() doesn't call
thp_get_unmapped_area() to get a THP aligned address.

To fix it, call get_unmapped_area() with the realfile.

Besides, since overlayfs may be built with CONFIG_OVERLAY_FS=m, we should
export get_unmapped_area().

Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
---
 fs/overlayfs/file.c | 20 ++++++++++++++++++++
 mm/mmap.c           |  1 +
 2 files changed, 21 insertions(+)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 969b458100fe..d0dcf675ebe8 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -653,6 +653,25 @@ static int ovl_flush(struct file *file, fl_owner_t id)
 	return err;
 }
 
+static unsigned long ovl_get_unmapped_area(struct file *file,
+		unsigned long addr, unsigned long len, unsigned long pgoff,
+		unsigned long flags)
+{
+	struct file *realfile;
+	const struct cred *old_cred;
+	unsigned long ret;
+
+	realfile = ovl_real_file(file);
+	if (IS_ERR(realfile))
+		return PTR_ERR(realfile);
+
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	ret = get_unmapped_area(realfile, addr, len, pgoff, flags);
+	ovl_revert_creds(old_cred);
+
+	return ret;
+}
+
 const struct file_operations ovl_file_operations = {
 	.open		= ovl_open,
 	.release	= ovl_release,
@@ -661,6 +680,7 @@ const struct file_operations ovl_file_operations = {
 	.write_iter	= ovl_write_iter,
 	.fsync		= ovl_fsync,
 	.mmap		= ovl_mmap,
+	.get_unmapped_area = ovl_get_unmapped_area,
 	.fallocate	= ovl_fallocate,
 	.fadvise	= ovl_fadvise,
 	.flush		= ovl_flush,
diff --git a/mm/mmap.c b/mm/mmap.c
index 16f8e8be01f8..60eb1ff7c9a8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -913,6 +913,7 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
 	error = security_mmap_addr(addr);
 	return error ? error : addr;
 }
+EXPORT_SYMBOL(__get_unmapped_area);
 
 unsigned long
 mm_get_unmapped_area(struct mm_struct *mm, struct file *file,
-- 
2.34.1