Re: [PATCH 1/3] xfs: introduce XFS_MAX_FILEOFF

[Dave Chinner <david@fromorbit.com>]

On Wed, Jan 08, 2020 at 02:32:38PM -0800, Darrick J. Wong wrote:
> On Thu, Jan 09, 2020 at 07:40:41AM +1100, Dave Chinner wrote:
> > On Tue, Jan 07, 2020 at 08:17:38PM -0800, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > Introduce a new #define for the maximum supported file block offset.
> > > We'll use this in the next patch to make it more obvious that we're
> > > doing some operation for all possible inode fork mappings after a given
> > > offset.  We can't use ULLONG_MAX here because bunmapi uses that to
> > > detect when it's done.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > >  fs/xfs/libxfs/xfs_format.h |    1 +
> > >  fs/xfs/xfs_reflink.c       |    3 ++-
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> > > index 1b7dcbae051c..c2976e441d43 100644
> > > --- a/fs/xfs/libxfs/xfs_format.h
> > > +++ b/fs/xfs/libxfs/xfs_format.h
> > > @@ -1540,6 +1540,7 @@ typedef struct xfs_bmdr_block {
> > >  #define BMBT_BLOCKCOUNT_BITLEN	21
> > >  
> > >  #define BMBT_STARTOFF_MASK	((1ULL << BMBT_STARTOFF_BITLEN) - 1)
> > > +#define XFS_MAX_FILEOFF		(BMBT_STARTOFF_MASK)
> > 
> > Isn't the maximum file offset in the BMBT the max start offset + the
> > max length of the extent that is located at BMBT_STARTOFF_MASK?
> 
> Apologies for responding to a question with another question, but has
> there ever been an XFS that supported an inode size of more than 8EB?

Doubt it.

> Linux supports at most a file offset of 8EB, which is 2^63-1, or
> 0x7FFF,FFFF,FFFF,FFFF.  On a filesystem with 512-byte blocks, the very
> last byte in the file would be in block 2^54-1, or 0x3F,FFFF,FFFF,FFFF.
> Larger blocksizes decrease that even further (e.g. 2^47-1, or
> 0x7FFF,FFFF,FFFF on 64k block filesystems).
>
> Therefore, on Linux I conclude that the largest file offset (block)
> possible is 2^54-1, which is BMBT_STARTOFF_MASK.  Unless there's an
> XFS port that actually supports 16EB files, BMBT_STARTOFF_MASK will
> suffice here.

Sure, but my point was that checks against the max file offset
as a block count are applied to the startoff field, not the
startoff + blockcount value, so we can potentially get extents on
disk beyond the above definition of XFS_MAX_FILEOFF...

i.e. startoff can be < XFS_MAX_FILEOFF, but startoff + blockcount
can be > XFS_MAX_FILEOFF, and there's nothing in the code that
prevents that from occurring...

e.g. what's preventing speculative delalloc from going beyond
XFS_MAX_FILEOFF, even though the actual file offset that is being
written is within XFS_MAX_FILEOFF?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com