drivers/char/mem: implement splice() for /dev/zero, /dev/full

This allows splicing zeroed pages into a pipe, and allows discarding
pages from a pipe by splicing them to /dev/zero.  Writing to /dev/zero
should have the same effect as writing to /dev/null, and a
"splice_write" implementation exists only for /dev/null.

(The /dev/zero splice_read implementation could be optimized by
pushing references to the global zero page to the pipe, but that's an
optimization for another day.)

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Link: https://lore.kernel.org/r/20230919073743.1066313-1-max.kellermann@ionos.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 1052b0f2d4cf86fed49218770a389669e6a3bcf2..263c19cd6fb959e7610deb719835e05a63201f33 100644 (file)
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -640,6 +640,7 @@ static int open_port(struct inode *inode, struct file *filp)
 #define full_lseek      null_lseek
 #define write_zero     write_null
 #define write_iter_zero        write_iter_null
+#define splice_write_zero      splice_write_null
 #define open_mem       open_port
 
 static const struct file_operations __maybe_unused mem_fops = {
@@ -677,6 +678,8 @@ static const struct file_operations zero_fops = {
        .read_iter      = read_iter_zero,
        .read           = read_zero,
        .write_iter     = write_iter_zero,
+       .splice_read    = copy_splice_read,
+       .splice_write   = splice_write_zero,
        .mmap           = mmap_zero,
        .get_unmapped_area = get_unmapped_area_zero,
 #ifndef CONFIG_MMU
@@ -688,6 +691,7 @@ static const struct file_operations full_fops = {
        .llseek         = full_lseek,
        .read_iter      = read_iter_zero,
        .write          = write_full,
+       .splice_read    = copy_splice_read,
 };
 
 static const struct memdev {