[PATCH] erofs: use the shared page cache for splice in inode_share mode

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng1024@gmail.com>

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

erofs_ishare_fops routes everything that touches the page cache to the
backing file in ->private_data: read_iter clones the iocb onto it, mmap
does vma_set_file(), fadvise calls vfs_fadvise() on it.  splice_read was
left as filemap_splice_read(), which works on the user file's own mapping.

filemap_splice_read() does init_sync_kiocb(&iocb, in), and
filemap_get_pages() then takes iocb->ki_filp->f_mapping, so splice() and
sendfile() populate the per-file page cache from disk instead of using the
shared one.  The data is correct, since erofs_fill_inode() sets that
mapping's a_ops either way, but the same content ends up cached twice,
which is what inode_share is there to avoid.

Pass the backing file, as read_iter already does.

Fixes: 5ef3208e3be5 ("erofs: introduce the page cache share feature")
Link: https://lore.kernel.org/all/b7dc7192-d586-45a2-bc4a-b41dc681c9bb@linux.alibaba.com/
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 fs/erofs/ishare.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/ishare.c b/fs/erofs/ishare.c
index fa7d4112dec5..01dc53e9e3ad 100644
--- a/fs/erofs/ishare.c
+++ b/fs/erofs/ishare.c
@@ -148,6 +148,13 @@ static int erofs_ishare_mmap(struct file *file, struct vm_area_struct *vma)
 	return generic_file_readonly_mmap(file, vma);
 }
 
+static ssize_t erofs_ishare_splice_read(struct file *in, loff_t *ppos,
+					struct pipe_inode_info *pipe,
+					size_t len, unsigned int flags)
+{
+	return filemap_splice_read(in->private_data, ppos, pipe, len, flags);
+}
+
 static int erofs_ishare_fadvise(struct file *file, loff_t offset,
 				loff_t len, int advice)
 {
@@ -161,7 +168,7 @@ const struct file_operations erofs_ishare_fops = {
 	.mmap		= erofs_ishare_mmap,
 	.release	= erofs_ishare_file_release,
 	.get_unmapped_area = thp_get_unmapped_area,
-	.splice_read	= filemap_splice_read,
+	.splice_read	= erofs_ishare_splice_read,
 	.fadvise	= erofs_ishare_fadvise,
 };
 
-- 
2.43.0