Re: [PATCH 5/7] xfs: Use 2^27 as the maximum number of directory extents

Chandan Babu R
2020-06-09T14:24:23Z

On Monday 8 June 2020 10:22:17 PM IST Darrick J. Wong wrote:
> On Sat, Jun 06, 2020 at 01:57:43PM +0530, Chandan Babu R wrote:
> > The maximum number of extents that can be used by a directory can be
> > calculated as shown below. (FS block size is assumed to be 512 bytes
> > since the smallest allowed block size can create a BMBT of maximum
> > possible height).
> >
> > Maximum number of extents in data space =
> > XFS_DIR2_SPACE_SIZE / 2^9 = 32GiB / 2^9 = 2^26.
> >
> > Maximum number (theoretically) of extents in leaf space =
> > 32GiB / 2^9 = 2^26.
>
> Hm. The leaf hash entries are 8 bytes long, whereas I think directory
> entries occupy at least 16 bytes. Is there a situation where the number
> of dir leaf/dabtree blocks can actually hit the 32G section size limit?

I don't think so. The 2^26 extents above was a theoretical limit. I wanted to
prove that even with the theoretical limit, the maximum number of extents used
by a directory is much less than 2^47 extents.

>
> > Maximum number of entries in a free space index block
> > = (512 - (sizeof struct xfs_dir3_free_hdr)) / (sizeof struct
> > xfs_dir2_data_off_t)
> > = (512 - 64) / 2 = 224
> >
> > Maximum number of extents in free space index =
> > (Maximum number of extents in data segment) / 224 =
> > 2^26 / 224 = ~2^18
> >
> > Maximum number of extents in a directory =
> > Maximum number of extents in data space +
> > Maximum number of extents in leaf space +
> > Maximum number of extents in free space index =
> > 2^26 + 2^26 + 2^18 = ~2^27
>
> I calculated the exact expression here, and got:
>
> 2^26 + 2^26 + (2^26/224) = 134,517,321
>
> This requires 28 bits of space, doesn't it?

You are right.

Log_2(134,517,321) returns 27.003. Since I had assumed a theoretical maximum
for the "leaf space extent count", I had rounded it down to 27 bits. I will
change this to 28 bits.

>
> Granted I bet the leaf section won't come within 300,000 nextents of the
> 2^26 you've assumed for it, so I suspect that in real world scenarios,
> 27 bits is enough. But if you're anticipating a totally full leaf
> section under extreme fragmentation, then MAXDIREXTNUM ought to be able
> to handle that.
>
> (Assuming I did any of that math correctly. ;))
>
> --D
>
> >
> > This commit defines the macro MAXDIREXTNUM to have the value 2^27 and
> > this in turn is used in calculating the maximum height of a directory
> > BMBT.
> >
> > Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> > ---
> > fs/xfs/libxfs/xfs_bmap.c | 2 +-
> > fs/xfs/libxfs/xfs_types.h | 1 +
> > 2 files changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 8b0029b3cecf..f75b70ae7b1f 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -81,7 +81,7 @@ xfs_bmap_compute_maxlevels(
> > if (whichfork == XFS_DATA_FORK) {
> > sz = XFS_BMDR_SPACE_CALC(MINDBTPTRS);
> > if (dir_bmbt)
> > - maxleafents = MAXEXTNUM;
> > + maxleafents = MAXDIREXTNUM;
> > else
> > maxleafents = MAXEXTNUM;
> > } else {
> > diff --git a/fs/xfs/libxfs/xfs_types.h b/fs/xfs/libxfs/xfs_types.h
> > index 397d94775440..0a3041ad5bec 100644
> > --- a/fs/xfs/libxfs/xfs_types.h
> > +++ b/fs/xfs/libxfs/xfs_types.h
> > @@ -60,6 +60,7 @@ typedef void * xfs_failaddr_t;
> > */
> > #define MAXEXTLEN ((xfs_extlen_t)0x001fffff) /* 21 bits */
> > #define MAXEXTNUM ((xfs_extnum_t)0x7fffffff) /* signed int */
> > +#define MAXDIREXTNUM ((xfs_extnum_t)0x7ffffff) /* 27 bits */
> > #define MAXAEXTNUM ((xfs_aextnum_t)0x7fff) /* signed short */
> >
> > /*
>

--
chandan