Re: [PATCH v3 00/18] mm/huge_memory: clean up folio split and lift swapcache split limits

[Yeoreum Yun]

> This series clean up the split code, add better swap cache split support
> for mappingless, large order, uniform and non-uniform split.  Generic
> performance is on par or slightly better, and stack usage is reduced.
> 
> The swap cache infrastructure can handle non-uniform or high order folio
> replace, so there is no reason for either restriction from the THP side.
> What stands in the way is the mixed anon/file folio split routine,
> which makes lifting the restrictions hard to follow, and it already
> carries some buggy or redundant checks.
> 
> So this series cleans up the split path and separates anon and file
> splitting into two helpers.  The file split path never sees a swap
> cache folio, and that is now enforced up front: a folio that is both
> in the page cache and the swap cache can only be a shmem folio, which
> remains unsupported and is rejected early.  That helps to rule out swap
> cache handling in that part completely.  Only the anon split path
> handles swap cache folios, with an anon mapping or mappingless:
> either way the splitting is similar, and non-uniform split is
> supported as well.
> 
> Order-1 is still forbidden for swap cache splitting.  In theory it is
> doable for shmem swap cache folios, but a mappingless swap cache
> folio cannot currently be told apart from a shmem one, so forbid it
> for all swap cache folios for now.
> 
> Testing:
> 
> The in-tree split_huge_page_test selftest (uniform, non-uniform and
> in-folio-offset splits of anon and pagecache folios) passes 62/62 on
> the patched kernel.
> 
> ftrace function_graph tracing filtered on __folio_split() was used to
> compare per-call durations between the base and the patched kernel on
> the same x86-64 box (interleaved runs across alternating reboots;
> 135 split calls per run, 50 test run):
> 
> Before: 67.9 us, stddev: 1.59
> After:  66.4 us, stddev: 1.19
> 
> The patched kernel is slightly faster. The stack usage is also reduced
> by about ~10%, with a very slight growth of huge_memory.o.
> 
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
> Changes in v3:
> - Get rid of for_each_folio_safe and open code it.
> - Check if the folio is mapped before freeing it swap cache to avoid
>   potential performance lose.
> - Initial test and binary analyze showed everything is very similiar to
>   previously series.
> - Drop the redundant mapping argument of __split_frozen_folio
> - Link to v2: https://patch.msgid.link/20260813-swap-thp-cleanup-v2-0-d2ee48c6aa49@xxxxxxxxxxx
> 
> Changes in v2:
> - Return -EBUSY instead of -EINVAL for swap cache & shmem folio split
>   attempt.
> - Introduce a for_each_folio_safe macro to dedupliate the code and
>   hightlight the reason we need to keep the iterate safe from folio
>   freeing. [ Zi Yan ]
> - Rename __split_unmapped_folio() to __split_frozen_folio [ Zi Yan ]
> - Rename __folio_freeze_split_unmap_anon. [ Zi Yan ]
> - Several comment improments [ Zi Yan ]
> - Drop an unused do_lru argument.
> - Previouse test results are basically unchanged, stack usage reduced,
>   object very slightly larger.
> - Link to v1: https://patch.msgid.link/20260808-swap-thp-cleanup-v1-0-689939a7ccc3@xxxxxxxxxxx
> 
> ---
> Kairui Song (18):
>       mm/swap: fix off-by-one in swap cache replace sanity check
>       mm/huge_memory: fix rejection of swap cache folios with a mapping
>       mm/huge_memory: invert folio_ref_freeze() check to reduce indentation
>       mm/huge_memory: split the routine for splitting anon and file folio
>       mm/huge_memory: rename __split_unmapped_folio() to __split_frozen_folio()
>       mm/huge_memory: consolidate irq and locking for folio split
>       mm/huge_memory: move EOF trimming into the file split helper
>       mm/huge_memory: move unmap and remap into the split helpers
>       mm/huge_memory: move anon_vma and filemap management into split helpers
>       mm/huge_memory: move memcg switch into the file split helper
>       mm/huge_memory: allow splitting mappingless swap cache folios
>       mm/huge_memory: add kerneldoc for the split helpers
>       mm/huge_memory: drop the unused do_lru argument of the file split helper
>       mm/huge_memory: clean up after-split folio freeing in __folio_split
>       mm/huge_memory: lift order-0 restriction for swapcache split
>       mm/huge_memory: clarify supported split orders in comment
>       mm/huge_memory: count only swap cache refs in anon folio split
>       mm/huge_memory: drop the redundant mapping argument of __split_frozen_folio
> 
>  mm/huge_memory.c | 633 +++++++++++++++++++++++++++++--------------------------
>  mm/swap_state.c  |   3 +-
>  2 files changed, 338 insertions(+), 298 deletions(-)
> ---
> base-commit: 4b2ae13f3393ef4b4bce0021e8762790354f369f
> change-id: 20260804-swap-thp-cleanup-6ce2be6cf3b8
> 
> Best regards,
> --  
> Kairui Song <kasong@xxxxxxxxxxx>

Nice cleanup. this series look good to me.

Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>

-- 
Sincerely,
Yeoreum Yun