[PATCH v5 0/4] allow partial folio write with iomap_folio_state

[alexjlzheng@gmail.com]

From: Jinliang Zheng <alexjlzheng@tencent.com>

Currently, if a partial write occurs in a buffer write, the entire write will
be discarded. While this is an uncommon case, it's still a bit wasteful and
we can do better.

With iomap_folio_state, we can identify uptodate states at the block
level, and a read_folio reading can correctly handle partially
uptodate folios.

Therefore, when a partial write occurs, accept the block-aligned
partial write instead of rejecting the entire write.

For example, suppose a folio is 2MB, blocksize is 4kB, and the copied
bytes are 2MB-3kB.

Without this patchset, we'd need to recopy from the beginning of the
folio in the next iteration, which means 2MB-3kB of bytes is copy
duplicately.

 |<-------------------- 2MB -------------------->|
 +-------+-------+-------+-------+-------+-------+
 | block |  ...  | block | block |  ...  | block | folio
 +-------+-------+-------+-------+-------+-------+
 |<-4kB->|

 |<--------------- copied 2MB-3kB --------->|       first time copied
 |<-------- 1MB -------->|                          next time we need copy (chunk /= 2)
                         |<-------- 1MB -------->|  next next time we need copy.

 |<------ 2MB-3kB bytes duplicate copy ---->|

With this patchset, we can accept 2MB-4kB of bytes, which is block-aligned.
This means we only need to process the remaining 4kB in the next iteration,
which means there's only 1kB we need to copy duplicately.

 |<-------------------- 2MB -------------------->|
 +-------+-------+-------+-------+-------+-------+
 | block |  ...  | block | block |  ...  | block | folio
 +-------+-------+-------+-------+-------+-------+
 |<-4kB->|

 |<--------------- copied 2MB-3kB --------->|       first time copied
                                         |<-4kB->|  next time we need copy

                                         |<>|
                              only 1kB bytes duplicate copy

Although partial writes are inherently a relatively unusual situation and do
not account for a large proportion of performance testing, the optimization
here still makes sense in large-scale data centers.

This patchset has been tested by xfstests' generic and xfs group, and
there's no new failed cases compared to the lastest upstream version kernel.

Changelog:

V5: patch[1]: use WARN_ON_ONCE() instead of WARN_ON(), suggested by Pankaj Raghav (Samsung)

V4: https://lore.kernel.org/linux-fsdevel/eyyshgzsxupyen6ms3izkh45ydh3ekxycpk5p4dbets6mpyhch@q4db2ayr4g3r/
    patch[4]: better documentation in code, and add motivation to the cover letter

V3: https://lore.kernel.org/linux-xfs/aMPIDGq7pVuURg1t@infradead.org/
    patch[1]: use WARN_ON() instead of BUG_ON()
    patch[2]: make commit message clear
    patch[3]: -
    patch[4]: make commit message clear

V2: https://lore.kernel.org/linux-fsdevel/20250810101554.257060-1-alexjlzheng@tencent.com/ 
    use & instead of % for 64 bit variable on m68k/xtensa, try to make them happy:
       m68k-linux-ld: fs/iomap/buffered-io.o: in function `iomap_adjust_read_range':
    >> buffered-io.c:(.text+0xa8a): undefined reference to `__moddi3'
    >> m68k-linux-ld: buffered-io.c:(.text+0xaa8): undefined reference to `__moddi3'

V1: https://lore.kernel.org/linux-fsdevel/20250810044806.3433783-1-alexjlzheng@tencent.com/

Jinliang Zheng (4):
  iomap: make sure iomap_adjust_read_range() are aligned with block_size
  iomap: move iter revert case out of the unwritten branch
  iomap: make iomap_write_end() return the number of written length again
  iomap: don't abandon the whole copy when we have iomap_folio_state

 fs/iomap/buffered-io.c | 80 +++++++++++++++++++++++++++++-------------
 1 file changed, 55 insertions(+), 25 deletions(-)

-- 
2.49.0