Commit 5500316 (May 2012) implemented raw_co_is_allocated() as
follows:
1. If defined(CONFIG_FIEMAP), use the FS_IOC_FIEMAP ioctl
2. Else if defined(SEEK_HOLE) && defined(SEEK_DATA), use lseek()
3. Else pretend there are no holes
Later on, raw_co_is_allocated() was generalized to
raw_co_get_block_status().
Commit 4f11aa8 (May 2014) changed it to try the three methods in order
until success, because "there may be implementations which support
[SEEK_HOLE/SEEK_DATA] but not [FIEMAP] (e.g., NFSv4.2) as well as vice
versa."
Unfortunately, we used FIEMAP incorrectly: we lacked FIEMAP_FLAG_SYNC.
Commit 38c4d0a (Sep 2014) added it. Because that's a significant
speed hit, the next commit 38c4d0a put SEEK_HOLE/SEEK_DATA first.
As you see, the obvious use of FIEMAP is wrong, and the correct use is
slow. I guess this puts it somewhere between -7 "The obvious use is
wrong" and -10 "It's impossible to get right" on Rusty Russel's Hard
to Misuse scale[*].
"Fortunately", the FIEMAP code is used only when
* SEEK_HOLE/SEEK_DATA arent't defined, but CONFIG_FIEMAP is
Uncommon. SEEK_HOLE had no XFS implementation between 2011 (when it
was introduced for ext4 and btrfs) and 2012.
* SEEK_HOLE/SEEK_DATA and CONFIG_FIEMAP are defined, but lseek() fails
Unlikely.
Thus, the FIEMAP code executes rarely. Makes it a nice hidey-hole for
bugs. Worse, bugs hiding there can theoretically bite even on a host
that has SEEK_HOLE/SEEK_DATA.
I don't want to worry about this crap, not even theoretically. Get
rid of it, then clean up the mess, including spotty error checking.
[*] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html
Signed-off-by: Markus Armbruster <address@hidden>
---
block/raw-posix.c | 128 ++++++++++++++++++++----------------------------------
1 file changed, 47 insertions(+), 81 deletions(-)
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 706d3c0..d16764c 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -60,9 +60,6 @@
#define FS_NOCOW_FL 0x00800000 /* Do not cow file */
#endif
#endif
-#ifdef CONFIG_FIEMAP
-#include <linux/fiemap.h>
-#endif
#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
#include <linux/falloc.h>
#endif
@@ -1481,77 +1478,56 @@ out:
return result;
}
-static int try_fiemap(BlockDriverState *bs, off_t start, off_t *data,
- off_t *hole, int nb_sectors)
+/*
+ * Find allocation range in @bs around offset @start.
+ * If @start is in a hole, store @start in @hole and the end of the
+ * hole in @data.
+ * If @start is in a data, store @start to @data, and the end of the
+ * data to @hole.
+ * If we can't find out, pretend there are no holes.
+ */
+static void find_allocation(BlockDriverState *bs, off_t start,
+ off_t *data, off_t *hole)
{
-#ifdef CONFIG_FIEMAP
+#if defined(SEEK_DATA) && defined(SEEK_HOLE)
BDRVRawState *s = bs->opaque;
- int ret = 0;
- struct {
- struct fiemap fm;
- struct fiemap_extent fe;
- } f;
+ off_t offs;
- if (s->skip_fiemap) {
- return -ENOTSUP;
+ offs = lseek(s->fd, start, SEEK_HOLE);
+ if (offs < 0) {
+ goto dunno;
}
+ assert(offs >= start);
- f.fm.fm_start = start;
- f.fm.fm_length = (int64_t)nb_sectors * BDRV_SECTOR_SIZE;
- f.fm.fm_flags = FIEMAP_FLAG_SYNC;
- f.fm.fm_extent_count = 1;
- f.fm.fm_reserved = 0;
- if (ioctl(s->fd, FS_IOC_FIEMAP, &f) == -1) {
- s->skip_fiemap = true;
- return -errno;
- }
-
- if (f.fm.fm_mapped_extents == 0) {
- /* No extents found, data is beyond f.fm.fm_start + f.fm.fm_length.
- * f.fm.fm_start + f.fm.fm_length must be clamped to the file size!
- */
- off_t length = lseek(s->fd, 0, SEEK_END);
- *hole = f.fm.fm_start;
- *data = MIN(f.fm.fm_start + f.fm.fm_length, length);
- } else {
- *data = f.fe.fe_logical;
- *hole = f.fe.fe_logical + f.fe.fe_length;
- if (f.fe.fe_flags & FIEMAP_EXTENT_UNWRITTEN) {
- ret |= BDRV_BLOCK_ZERO;
- }
- }
-
- return ret;
-#else
- return -ENOTSUP;
-#endif
-}
-
-static int try_seek_hole(BlockDriverState *bs, off_t start, off_t *data,
- off_t *hole)
-{
-#if defined SEEK_HOLE && defined SEEK_DATA
- BDRVRawState *s = bs->opaque;
-
- *hole = lseek(s->fd, start, SEEK_HOLE);
- if (*hole == -1) {
- return -errno;
- }
-
- if (*hole > start) {
+ if (offs > start) {
+ /* in data, next hole at offs */
*data = start;
- } else {
- /* On a hole. We need another syscall to find its end. */
- *data = lseek(s->fd, start, SEEK_DATA);
- if (*data == -1) {
- *data = lseek(s->fd, 0, SEEK_END);
- }
+ *hole = offs;
+ return;
}
- return 0;
-#else
- return -ENOTSUP;
+ /* in hole, end not yet known */
+ offs = lseek(s->fd, start, SEEK_DATA);
+ if (offs < 0) {
+ /* no idea where the hole ends, give up (unlikely to happen) */
+ goto dunno;
+ }
+ assert(offs >= start);
+ *hole = start;
+ *data = offs;
+ return;
+
+dunno:
#endif
+ /* assume all data */
+ offs = lseek(s->fd, 0, SEEK_END);
+ if (offs < 0) {
+ /* now that's *really* unexpected */
+ offs = (off_t)1 << (sizeof(off_t) * 8 - 1);
+ offs += offs - 1;
+ }
+ *data = start;
+ *hole = offs;
}
/*
@@ -1591,28 +1567,18 @@ static int64_t coroutine_fn
raw_co_get_block_status(BlockDriverState *bs,
nb_sectors = DIV_ROUND_UP(total_size - start, BDRV_SECTOR_SIZE);
}
- ret = try_seek_hole(bs, start, &data, &hole);
- if (ret < 0) {
- ret = try_fiemap(bs, start, &data, &hole, nb_sectors);
- if (ret < 0) {
- /* Assume everything is allocated. */
- data = 0;
- hole = start + nb_sectors * BDRV_SECTOR_SIZE;
- ret = 0;
- }
- }
-
- assert(ret >= 0);
-
- if (data <= start) {
+ ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start;
+ find_allocation(bs, start, &data, &hole);
+ if (data == start) {
/* On a data extent, compute sectors to the end of the extent. */
*pnum = MIN(nb_sectors, (hole - start) / BDRV_SECTOR_SIZE);
- return ret | BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start;
} else {
/* On a hole, compute sectors to the beginning of the next extent. */
+ assert(hole == start);
*pnum = MIN(nb_sectors, (data - start) / BDRV_SECTOR_SIZE);
- return ret | BDRV_BLOCK_ZERO | BDRV_BLOCK_OFFSET_VALID | start;
+ ret |= BDRV_BLOCK_ZERO;