[RFC PATCH 4/4] splice: Record some statistics

[David Howells <dhowells@xxxxxxxxxx>]

Add a proc file to export some statistics for debugging purposes.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: Dave Chinner <david@xxxxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/splice.c            | 28 ++++++++++++++++++++++++++++
 include/linux/splice.h |  3 +++
 mm/filemap.c           |  6 +++++-
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/fs/splice.c b/fs/splice.c
index 2b1f109a7d4f..831973ea6b3f 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -36,10 +36,15 @@
 #include <linux/net.h>
 #include <linux/socket.h>
 #include <linux/sched/signal.h>
+#include <linux/proc_fs.h>
 
 #include "../mm/internal.h"
 #include "internal.h"
 
+atomic_t splice_stat_filemap_copied, splice_stat_filemap_moved;
+static atomic_t splice_stat_directly_copied;
+static atomic_t vmsplice_stat_copied, vmsplice_stat_stole;
+
 /*
  * Splice doesn't support FMODE_NOWAIT. Since pipes may set this flag to
  * indicate they support non-blocking reads or writes, we must clear it
@@ -276,6 +281,7 @@ ssize_t copy_splice_read(struct file *in, loff_t *ppos,
 		remain -= chunk;
 	}
 
+	atomic_add(keep, &splice_stat_directly_copied);
 	kfree(bv);
 	return ret;
 }
@@ -1299,6 +1305,7 @@ static int splice_try_to_steal_page(struct pipe_inode_info *pipe,
 	unmap_mapping_folio(folio);
 	if (remove_mapping(folio->mapping, folio)) {
 		folio_clear_mappedtodisk(folio);
+		atomic_inc(&vmsplice_stat_stole);
 		flags |= PIPE_BUF_FLAG_LRU;
 		goto add_to_pipe;
 	}
@@ -1316,6 +1323,7 @@ static int splice_try_to_steal_page(struct pipe_inode_info *pipe,
 	folio_put(folio);
 	folio = copy;
 	offset = 0;
+	atomic_inc(&vmsplice_stat_copied);
 
 add_to_pipe:
 	page = folio_page(folio, offset / PAGE_SIZE);
@@ -1905,3 +1913,23 @@ SYSCALL_DEFINE4(tee, int, fdin, int, fdout, size_t, len, unsigned int, flags)
 
 	return error;
 }
+
+static int splice_stats_show(struct seq_file *m, void *data)
+{
+	seq_printf(m, "filemap: copied=%u moved=%u\n",
+		   atomic_read(&splice_stat_filemap_copied),
+		   atomic_read(&splice_stat_filemap_moved));
+	seq_printf(m, "direct : copied=%u\n",
+		   atomic_read(&splice_stat_directly_copied));
+	seq_printf(m, "vmsplice: copied=%u stole=%u\n",
+		   atomic_read(&vmsplice_stat_copied),
+		   atomic_read(&vmsplice_stat_stole));
+	return 0;
+}
+
+static int splice_stats_init(void)
+{
+	proc_create_single("fs/splice", S_IFREG | 0444, NULL, splice_stats_show);
+	return 0;
+}
+late_initcall(splice_stats_init);
diff --git a/include/linux/splice.h b/include/linux/splice.h
index 3c5abbd49ff2..4f04dc338010 100644
--- a/include/linux/splice.h
+++ b/include/linux/splice.h
@@ -98,4 +98,7 @@ extern int splice_grow_spd(const struct pipe_inode_info *, struct splice_pipe_de
 extern void splice_shrink_spd(struct splice_pipe_desc *);
 
 extern const struct pipe_buf_operations default_pipe_buf_ops;
+
+extern atomic_t splice_stat_filemap_copied, splice_stat_filemap_moved;
+
 #endif
diff --git a/mm/filemap.c b/mm/filemap.c
index dd144b0dab69..38d38cc826fa 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2872,7 +2872,8 @@ ssize_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 	struct address_space *mapping;
 	struct folio *copy = NULL;
 	struct page *page;
-	unsigned int flags = 0;
+	unsigned int flags = 0, count = 0;
+	atomic_t *stat = &splice_stat_filemap_copied;
 	ssize_t ret;
 	size_t spliced = 0, offset = offset_in_folio(folio, fpos);
 
@@ -2902,6 +2903,7 @@ ssize_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 	/* If we succeed in removing the mapping, set LRU flag and add it. */
 	if (remove_mapping(mapping, folio)) {
 		folio_unlock(folio);
+		stat = &splice_stat_filemap_moved;
 		flags = PIPE_BUF_FLAG_LRU;
 		goto add_to_pipe;
 	}
@@ -2940,8 +2942,10 @@ ssize_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 		page++;
 		spliced += part;
 		offset = 0;
+		count++;
 	}
 
+	atomic_add(count, stat);
 	if (copy)
 		folio_put(copy);
 	return spliced;