[RFC PATCH 0/2] remove SWAP_MAP_SHMEM

[Nhat Pham]

The SWAP_MAP_SHMEM state was originally introduced in the commit
aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a
swap entry belongs to shmem during swapoff.

However, swapoff has since been rewritten drastically in the commit
b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now
having swap count == SWAP_MAP_SHMEM value is basically the same as having
swap count == 1, and swap_shmem_alloc() behaves analogously to
swap_duplicate()
    
This RFC proposes the removal of this state and the associated helper to
simplify the state machine (both mentally and code-wise). We will also
have an extra state/special value that can be repurposed (for swap entries
that never gets re-duplicated).

Another motivation (albeit a bit premature at the moment) is the new swap
abstraction I am currently working on, that would allow for swap/zswap
decoupling, swapoff optimization, etc. The fewer states and swap API
functions there are, the simpler the conversion will be.

I am sending this series first as an RFC, just in case I missed something
or misunderstood this state, or if someone has a swap optimization in mind
for shmem that would require this special state.

Swap experts, let me know if I'm mistaken :) Otherwise if there is no
objection I will resend this patch series again for merging.

Nhat Pham (2):
  swapfile: add a batched variant for swap_duplicate()
  swap: shmem: remove SWAP_MAP_SHMEM

 include/linux/swap.h | 16 ++++++++--------
 mm/shmem.c           |  2 +-
 mm/swapfile.c        | 28 +++++++++-------------------
 3 files changed, 18 insertions(+), 28 deletions(-)


base-commit: acfabf7e197f7a5bedf4749dac1f39551417b049
-- 
2.43.5