Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Sep 24, 2024 at 6:53 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
> One benefit I can mention is that removing 'SWAP_MAP_SHMEM' can help to
> batch free shmem swap entries in __swap_entries_free(), similar to the
> commit bea67dcc5eea ("mm: attempt to batch free swap entries for
> zap_pte_range()") did, which can improve the performance of shmem mTHP
> munmap() function based on my testing.

Yeah, the problem with having an extraneous state is you have to
constantly check for it in code, and/or keep it in mind when you
develop things. I've been constantly having to check for this state
when I develop code around this area, and it gets old fast.

If we can use it to optimize something, I can understand keeping it.
But it just seems like dead code to me :)

My preference is to do this as simply as possible - add another case
(usage == 1, nr > 1, and we need to add swap continuations) in the
check in __swap_duplicate()'s first loop, and just WARN right there.

That case CANNOT happen UNLESS we introduce a bug, or have a new use
case. When we actually have a use case, we can always introduce
handling/fallback logic for that case.

Barry, Yosry, Baolin, Ying, how do you feel about this?