Re: [PATCH] btrfs: zoned: fix bitops api misuse

[David Sterba <dsterba@suse.cz>]

On Thu, Apr 20, 2023 at 01:14:30PM +0900, Naohiro Aota wrote:
> On Thu, Apr 20, 2023 at 12:58:14PM +0900, Naohiro Aota wrote:
> > On Wed, Apr 19, 2023 at 01:24:56AM +0200, David Sterba wrote:
> > > On Tue, Apr 18, 2023 at 05:45:24PM +0900, Naohiro Aota wrote:
> > > > From: Naohiro Aota <naohiro.aota@wdc.com>
> > > > 
> > > > find_next_bit and find_next_zero_bit take @size as the second parameter and
> > > > @offset as the third parameter. They are specified opposite in
> > > > btrfs_ensure_empty_zones(). Thanks to the later loop, it never failed to
> > > > detect the empty zones. Fix them and (maybe) return the result a bit
> > > > faster.
> > > > 
> > > > Fixes: 1cd6121f2a38 ("btrfs: zoned: implement zoned chunk allocator")
> > > > CC: stable@vger.kernel.org # 5.15+
> > > > Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> > > > ---
> > > >  fs/btrfs/zoned.c | 6 +++---
> > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> > > > index 2b160fda7301..55bde1336d81 100644
> > > > --- a/fs/btrfs/zoned.c
> > > > +++ b/fs/btrfs/zoned.c
> > > > @@ -1171,12 +1171,12 @@ int btrfs_ensure_empty_zones(struct btrfs_device *device, u64 start, u64 size)
> > > >  		return -ERANGE;
> > > >  
> > > >  	/* All the zones are conventional */
> > > > -	if (find_next_bit(zinfo->seq_zones, begin, end) == end)
> > > > +	if (find_next_bit(zinfo->seq_zones, end, begin) == end)
> > > 
> > > End is defined as "end = (start + size) >> shift", and the 2nd parameter
> > > of find_next_bit is supposed to be 'size'. Shouldn't it be "size >>
> > > shift"?
> > 
> > Not so. The argument "size" represents the size of the allocation range,
> > which is to be confirmed as empty. OTOH, find_next_bit()'s "size" (the 2nd
> > parameter) represents the size of an entire bitmap, not the number of bits to
> > be tested.
> 
> BTW, I found the same logic is implemented in subpage.c as
> bitmap_test_range_all_{set,zero}. So, it might worth moving them to
> somewhere (misc.h?) and use them here.

Good idea, please send it as a separate patch. Thanks.