Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/9/24 07:11, Nhat Pham wrote:
> 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.

The idea makes sense to me. I did a quick test with shmem mTHP, and 
encountered the following warning which is triggered by 
'VM_WARN_ON(usage == 1 && nr > 1)' in __swap_duplicate().

[   81.064967] ------------[ cut here ]------------
[   81.064968] WARNING: CPU: 4 PID: 6852 at mm/swapfile.c:3623 
__swap_duplicate+0x1d0/0x2e0
[   81.064994] pstate: 23400005 (nzCv daif +PAN -UAO +TCO +DIT -SSBS 
BTYPE=--)
[   81.064995] pc : __swap_duplicate+0x1d0/0x2e0
[   81.064997] lr : swap_duplicate_nr+0x30/0x70
[......]
[   81.065019] Call trace:
[   81.065019]  __swap_duplicate+0x1d0/0x2e0
[   81.065021]  swap_duplicate_nr+0x30/0x70
[   81.065022]  shmem_writepage+0x24c/0x438
[   81.065024]  pageout+0x104/0x2e0
[   81.065026]  shrink_folio_list+0x7f0/0xe60
[   81.065027]  reclaim_folio_list+0x90/0x178
[   81.065029]  reclaim_pages+0x128/0x1a8
[   81.065030]  madvise_cold_or_pageout_pte_range+0x80c/0xd10
[   81.065031]  walk_pmd_range.isra.0+0x1b8/0x3a0
[   81.065033]  walk_pud_range+0x120/0x1b0
[   81.065035]  walk_pgd_range+0x150/0x1a8
[   81.065036]  __walk_page_range+0xa4/0xb8
[   81.065038]  walk_page_range+0x1c8/0x250
[   81.065039]  madvise_pageout+0xf4/0x280
[   81.065041]  madvise_vma_behavior+0x268/0x3f0
[   81.065042]  madvise_walk_vmas.constprop.0+0xb8/0x128
[   81.065043]  do_madvise.part.0+0xe8/0x2a0
[   81.065044]  __arm64_sys_madvise+0x64/0x78
[   81.065046]  invoke_syscall.constprop.0+0x54/0xe8
[   81.065048]  do_el0_svc+0xa4/0xc0
[   81.065050]  el0_svc+0x2c/0xb0
[   81.065052]  el0t_64_sync_handler+0xb8/0xc0
[   81.065054]  el0t_64_sync+0x14c/0x150

> 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