On Jun 14, 2023, at 11:48 PM, Christoph Hellwig [off-list ref] wrote:

The support for write intent bitmaps in files on an external files in md
is a hot mess that abuses ->bmap to map file offsets into physical device
objects, and also abuses buffer_heads in a creative way.

Make this code optional so that MD can be built into future kernels
without buffer_head support, and so that we can eventually deprecate it.

Note this does not affect the internal bitmap support, which has none of
the problems.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
drivers/md/Kconfig     | 10 ++++++++++
drivers/md/md-bitmap.c | 15 +++++++++++++++
drivers/md/md.c        |  7 +++++++
3 files changed, 32 insertions(+)

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index b0a22e99bade37..9712ab9bcba52e 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig

@@ -50,6 +50,16 @@ config MD_AUTODETECT

 If unsure, say Y.

+config MD_BITMAP_FILE
+ bool "MD bitmap file support"
+ default y
+ help
+  If you say Y here, support for write intent bitmaps in files on an
+  external file system is enabled.  This is an alternative to the internal
+  bitmaps near the MD superblock, and very problematic code that abuses
+  various kernel APIs and can only work with files on a file system not
+  actually sitting on the MD device.
+

config MD_LINEAR
tristate "Linear (append) mode (deprecated)"
depends on BLK_DEV_MD

diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
index ed402f4dad182d..1e29088b1f081a 100644
--- a/drivers/md/md-bitmap.c
+++ b/drivers/md/md-bitmap.c

@@ -295,6 +295,7 @@ static void write_sb_page(struct bitmap *bitmap, unsigned long pg_index,

static void md_bitmap_file_kick(struct bitmap *bitmap);

+#ifdef CONFIG_MD_BITMAP_FILE
static void write_file_page(struct bitmap *bitmap, struct page *page, int wait)
{
struct buffer_head *bh = page_buffers(page);

@@ -408,6 +409,20 @@ static int read_file_page(struct file *file, unsigned long index,
      ret);

return ret;
}
+#else /* CONFIG_MD_BITMAP_FILE */
+static void write_file_page(struct bitmap *bitmap, struct page *page, int wait)
+{
+}
+static int read_file_page(struct file *file, unsigned long index,
+ struct bitmap *bitmap, unsigned long count, struct page *page)
+{
+ return -EIO;
+}
+static void free_buffers(struct page *page)
+{
+ put_page(page);
+}
+#endif /* CONFIG_MD_BITMAP_FILE */

/*
 * bitmap file superblock operations

diff --git a/drivers/md/md.c b/drivers/md/md.c
index cf3733c90c47ed..c9fcefaf9c073b 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -7020,6 +7020,13 @@ static int set_bitmap_file(struct mddev *mddev, int fd)

if (mddev->bitmap || mddev->bitmap_info.file)
return -EEXIST; /* cannot add when bitmap is present */
+
+ if (!IS_ENABLED(CONFIG_MD_BITMAP_FILE)) {
+ pr_warn("%s: bitmap files not supported by this kernel\n",
+ mdname(mddev));
+ return -EINVAL;
+ }
+
f = fget(fd);

if (f == NULL) {
-- 
2.39.2