• Hans van Kranenburg's avatar
    btrfs: alloc_chunk: fix DUP stripe size handling · 92e222df
    Hans van Kranenburg authored
    In case of using DUP, we search for enough unallocated disk space on a
    device to hold two stripes.
    
    The devices_info[ndevs-1].max_avail that holds the amount of unallocated
    space found is directly assigned to stripe_size, while it's actually
    twice the stripe size.
    
    Later on in the code, an unconditional division of stripe_size by
    dev_stripes corrects the value, but in the meantime there's a check to
    see if the stripe_size does not exceed max_chunk_size. Since during this
    check stripe_size is twice the amount as intended, the check will reduce
    the stripe_size to max_chunk_size if the actual correct to be used
    stripe_size is more than half the amount of max_chunk_size.
    
    The unconditional division later tries to correct stripe_size, but will
    actually make sure we can't allocate more than half the max_chunk_size.
    
    Fix this by moving the division by dev_stripes before the max chunk size
    check, so it always contains the right value, instead of putting a duct
    tape division in further on to get it fixed again.
    
    Since in all other cases than DUP, dev_stripes is 1, this change only
    affects DUP.
    
    Other attempts in the past were made to fix this:
    * 37db63a4 "Btrfs: fix max chunk size check in chunk allocator" tried
    to fix the same problem, but still resulted in part of the code acting
    on a wrongly doubled stripe_size value.
    * 86db2578 "Btrfs: fix max chunk size on raid5/6" unintentionally
    broke this fix again.
    
    The real problem was already introduced with the rest of the code in
    73c5de00.
    
    The user visible result however will be that the max chunk size for DUP
    will suddenly double, while it's actually acting according to the limits
    in the code again like it was 5 years ago.
    Reported-by: default avatarNaohiro Aota <naohiro.aota@wdc.com>
    Link: https://www.spinics.net/lists/linux-btrfs/msg69752.html
    Fixes: 73c5de00 ("btrfs: quasi-round-robin for chunk allocation")
    Fixes: 86db2578 ("Btrfs: fix max chunk size on raid5/6")
    Signed-off-by: default avatarHans van Kranenburg <hans.van.kranenburg@mendix.com>
    Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
    [ update comment ]
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    92e222df
volumes.c 194 KB