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

[Fam Zheng]

On Wed, 11/12 20:27, Markus Armbruster wrote:
> 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.

s/38c4d0a/7c159037/

> 
> 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[*].

Nice reading :)

> 
> "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
> 
> 

Other than the wrong commit id in message,

Reviewed-by: Fam Zheng <address@hidden>