Re: [PATCH V5 13/16] xfs: Conditionally upgrade existing inodes to use 64-bit extent counters

Dave Chinner
2022-02-16T03:59:58Z

On Tue, Feb 15, 2022 at 05:16:33PM -0800, Darrick J. Wong wrote:
> On Tue, Feb 15, 2022 at 06:46:16PM +0530, Chandan Babu R wrote:
> > On 15 Feb 2022 at 17:03, Chandan Babu R wrote:
> > > On 15 Feb 2022 at 15:03, Dave Chinner wrote:
> > >> On Tue, Feb 15, 2022 at 12:18:50PM +0530, Chandan Babu R wrote:
> > >>> On 14 Feb 2022 at 22:37, Darrick J. Wong wrote:
> > >>> > On Fri, Feb 11, 2022 at 05:40:30PM +0530, Chandan Babu R wrote:
> > >>> >> On 07 Feb 2022 at 22:41, Darrick J. Wong wrote:
> > >>> >> > On Mon, Feb 07, 2022 at 10:25:19AM +0530, Chandan Babu R wrote:
> > >>> >> >> On 02 Feb 2022 at 01:31, Darrick J. Wong wrote:
> > >>> >> >> > On Fri, Jan 21, 2022 at 10:48:54AM +0530, Chandan Babu R wrote:
> > >>> >> >> I went through all the call sites of xfs_iext_count_may_overflow() and I think
> > >>> >> >> that your suggestion can be implemented.
> > >>> >>
> > >>> >> Sorry, I missed/overlooked the usage of xfs_iext_count_may_overflow() in
> > >>> >> xfs_symlink().
> > >>> >>
> > >>> >> Just after invoking xfs_iext_count_may_overflow(), we execute the following
> > >>> >> steps,
> > >>> >>
> > >>> >> 1. Allocate inode chunk
> > >>> >> 2. Initialize inode chunk.
> > >>> >> 3. Insert record into inobt/finobt.
> > >>> >> 4. Roll the transaction.
> > >>> >> 5. Allocate ondisk inode.
> > >>> >> 6. Add directory inode to transaction.
> > >>> >> 7. Allocate blocks to store symbolic link path name.
> > >>> >> 8. Log symlink's inode (data fork contains block mappings).
> > >>> >> 9. Log data blocks containing symbolic link path name.
> > >>> >> 10. Add name to directory and log directory's blocks.
> > >>> >> 11. Log directory inode.
> > >>> >> 12. Commit transaction.
> > >>> >>
> > >>> >> xfs_trans_roll() invoked in step 4 would mean that we cannot move step 6 to
> > >>> >> occur before step 1 since xfs_trans_roll would unlock the inode by executing
> > >>> >> xfs_inode_item_committing().
> > >>> >>
> > >>> >> xfs_create() has a similar flow.
> > >>> >>
> > >>> >> Hence, I think we should retain the current logic of setting
> > >>> >> XFS_DIFLAG2_NREXT64 just after reading the inode from the disk.
> > >>> >
> > >>> > File creation shouldn't ever run into problems with
> > >>> > xfs_iext_count_may_overflow because (a) only symlinks get created with
> > >>> > mapped blocks, and never more than two; and (b) we always set NREXT64
> > >>> > (the inode flag) on new files if NREXT64 (the superblock feature bit) is
> > >>> > enabled, so a newly created file will never require upgrading.
> > >>>
> > >>> The inode representing the symbolic link being created cannot overflow its
> > >>> data fork extent count field. However, the inode representing the directory
> > >>> inside which the symbolic link entry is being created, might overflow its data
> > >>> fork extent count field.
> > >>
> > >> I dont' think that can happen. A directory is limited in size to 3
> > >> segments of 32GB each. In reality, only the data segment can ever
> > >> reach 32GB as both the dabtree and free space segments are just
> > >> compact indexes of the contents of the 32GB data segment.
> > >>
> > >> Hence a directory is never likely to reach more than about 40GB of
> > >> blocks which is nowhere near large enough to overflowing a 32 bit
> > >> extent count field.
> > >
> > > I think you are right.
> > >
> > > The maximum file size that can be represented by the data fork extent counter
> > > in the worst case occurs when all extents are 1 block in size and each block
> > > is 1k in size.
> > >
> > > With 1k byte sized blocks, a file can reach upto,
> > > 1k * (2^31) = 2048 GB
> > >
> > > This is much larger than the asymptotic maximum size of a directory i.e.
> > > 32GB * 3 = 96GB.
>
> The downside of getting rid of the checks for directories is that we
> won't be able to use the error injection knob that limits all forks to
> 10 extents max to see what happens when that part of directory expansion
> fails. But if it makes it easier to handle nrext64, then that's
> probably a good enough reason to forego that.

If you want error injection to do that, add explicit error injection
to the directory code.

> > xfs_bmap_del_extent_real()
>
> Not sure about this one, since it actually /can/ result in more extents.

Yup, unlikely to ever trigger, but still necessary for correctness.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com