Re: [PATCH v2] mm: don't be stuck to rmap lock on reclaim path

[Andrew Morton]

On Mon,  9 May 2022 08:47:10 -0700 Minchan Kim <minchan@xxxxxxxxxx> wrote:

> The rmap locks(i_mmap_rwsem and anon_vma->root->rwsem) could be
> contented under memory pressure if processes keep working on
> their vmas(e.g., fork, mmap, munmap). It makes reclaim path
> stuck. In our real workload traces, we see kswapd is waiting the
> lock for 300ms+(a sec as worst case) and it makes other processes
> entering direct reclaim, which were also stuck on the lock.
> 
> This patch makes LRU aging path try_lock mode like shink_page_list
> so the reclaim context will keep working with next LRU pages
> without being stuck.
> 
> Since this patch introduces a new "contended" field as out-param
> along with try_lock in-param in rmap_walk_control, it's not
> immutable any longer if the try_lock is set so remove const
> keywords on rmap related functions. Since rmap walking is already
> expensive operation, I doubt the const would help sizable benefit(
> And we didn't have it until 5.17).

Some quantitative testing results would be helpful.  Demonstrate
the benefits of the patch?