[Qemu-devel] [PATCH 2/2] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP

[Markus Armbruster]

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;
     }
+    return ret;
 }
 
 static coroutine_fn BlockAIOCB *raw_aio_discard(BlockDriverState *bs,
-- 
1.9.3