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From 726f113a3fbea10e910a2962fa25b6274314380c Mon Sep 17 00:00:00 2001
From: Max Reitz <mreitz@redhat.com>
Date: Tue, 18 Nov 2014 15:30:19 +0100
Subject: [PATCH 40/41] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP

Message-id: <1416324620-16229-7-git-send-email-mreitz@redhat.com>
Patchwork-id: 62441
O-Subject: [RHEL-7.1/7.0.z qemu-kvm PATCH v3 6/7] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP
Bugzilla: 1160237
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Markus Armbruster <armbru@redhat.com>

From: Markus Armbruster <armbru@redhat.com>

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 7c159037 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 aren'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.

[*] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit c4875e5b2216cf5427459e619b10f75083565792)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

Conflicts:
	block/raw-posix.c

Upstream has a "needs_alignment" field in BDRVRawState, downstream does
not.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/raw-posix.c | 63 ++++---------------------------------------------------
 1 file changed, 4 insertions(+), 59 deletions(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index a46e50b..aeb8a97 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -57,9 +57,6 @@
 #include <linux/fd.h>
 #include <linux/fs.h>
 #endif
-#ifdef CONFIG_FIEMAP
-#include <linux/fiemap.h>
-#endif
 #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
 #include <linux/falloc.h>
 #endif
@@ -147,9 +144,6 @@ typedef struct BDRVRawState {
     bool has_discard:1;
     bool has_write_zeroes:1;
     bool discard_zeroes:1;
-#ifdef CONFIG_FIEMAP
-    bool skip_fiemap;
-#endif
 } BDRVRawState;
 
 typedef struct BDRVRawReopenState {
@@ -1308,52 +1302,6 @@ out:
     return result;
 }
 
-static int try_fiemap(BlockDriverState *bs, off_t start, off_t *data,
-                      off_t *hole, int nb_sectors)
-{
-#ifdef CONFIG_FIEMAP
-    BDRVRawState *s = bs->opaque;
-    int ret = 0;
-    struct {
-        struct fiemap fm;
-        struct fiemap_extent fe;
-    } f;
-
-    if (s->skip_fiemap) {
-        return -ENOTSUP;
-    }
-
-    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)
 {
@@ -1422,13 +1370,10 @@ static int64_t coroutine_fn raw_co_get_block_status(BlockDriverState *bs,
 
     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;
-        }
+        /* Assume everything is allocated. */
+        data = 0;
+        hole = start + nb_sectors * BDRV_SECTOR_SIZE;
+        ret = 0;
     }
 
     assert(ret >= 0);
-- 
1.8.3.1