[PATCH v5 0/4] xfs: seek_data/seek_hole refinements

[Jeff Liu <jeff.liu@oracle.com>]

Hello,

This is the v5 patch set for xfs_seek_data/xfs_seek_hole refinements with lookup the
offset from page cache for desired type.

I split those changes to 4 patches this time, maybe it could facilitate the review process.

v4->v5:
* Introduce a new routine xfs_find_get_desired_pgoff() to search the desired offset from page cache
  for unwritten and delayed-allocation extents.  This function is mainly based on the original xfs_has_unwritten_buffer() but
  revised to be able to fetch a hole offset directly for the refinement of xfs_seek_hole().
* Remove type argument from xfs_seek_data()/xfs_seek_hole() as it is already indicated by those function naming explicitly.

v3->v4:
xfs_seek_hole() refinement, suggested by Mark and Christoph.
* refine xfs_seek_hole() with unwritten extents search, treat it as a hole if no data buffer was found from page cache.
* s/goto out/break/g, break out of the extent maps reading loop rather than 'go to', I must have got my head up in the clouds when writing v3.
* xfs_has_unwritten_buffer(), remove 'offset <= XFS_FSB_TO_B(mp, last))' from BH state checking branch.
  The page index offset might less than '*start', so we will miss a data extent if so.
* xfs_has_unwritten_buffer(), don't reset '*offset' to ZERO if no data buffer was found because of xfs_seek_hole() will call this function to
  examine an unwritten extent has data or not. If not, it will use the returned '*offset' as a hole offset.  So set '*offset' to zero in
  xfs_has_unwritten_buffer() will lead to wrong result.
* avoid re-starting the next round search in both xfs_seek_data() and xfs_seek_hole() if the end offset of the 2nd extent map is hit the EOF.
  So for SEEK_DATA, it means there is no data extent beyond the current offset and return ENXIO, for SEEK_HOLE, return the file size to
  indicate hitting EOF.   The comments were also changed(s/reading offset not beyond/reading offset not beyond or hit EOF/)accordingly.

v2->v3:
Tested by Mark, hit BUG() for continuous unwritten extents without data wrote.
* xfs_seek_data(), remove BUG() and having extents map search in loop.

v1->v2:
suggested by Mark.
* xfs_has_unwritten_buffer(), use the input offset instead of bmap->br_startoff to calculate page index for data buffer probing.

Thanks,
-Jeff

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