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 <arm...@redhat.com> --- 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
