Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than clearing young

[SeongJae Park <sj@kernel.org>]

On Fri, 23 Feb 2024 17:15:50 +1300 Barry Song <21cnbao@gmail.com> wrote:

> From: Barry Song <v-songbaohua@oppo.com>
> 
> While doing MADV_PAGEOUT, the current code will clear PTE young
> so that vmscan won't read young flags to allow the reclamation
> of madvised folios to go ahead.
> It seems we can do it by directly ignoring references, thus we
> can remove tlb flush in madvise and rmap overhead in vmscan.
> 
> Regarding the side effect, in the original code, if a parallel
> thread runs side by side to access the madvised memory with the
> thread doing madvise, folios will get a chance to be re-activated
> by vmscan. But with the patch, they will still be reclaimed. But
> this behaviour doing PAGEOUT and doing access at the same time is
> quite silly like DoS. So probably, we don't need to care.

I think we might need to take care of the case, since users may use just a
best-effort estimation like DAMON for the target pages.  In such cases, the
page granularity re-check of the access could be helpful.  So I concern if this
could be a visible behavioral change for some valid use cases.

> 
> A microbench as below has shown 6% decrement on the latency of
> MADV_PAGEOUT,

I assume some of the users may use MADV_PAGEOUT for proactive reclamation of
the memory.  In the use case, I think latency of MADV_PAGEOUT might be not that
important.

Hence I think the cons of the behavioral change might outweigh the pros of the
latench improvement, for such best-effort proactive reclamation use case.  Hope
to hear and learn from others' opinions.

> 
>  #define PGSIZE 4096
>  main()
>  {
>  	int i;
>  #define SIZE 512*1024*1024
>  	volatile long *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
>  			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 
>  	for (i = 0; i < SIZE/sizeof(long); i += PGSIZE / sizeof(long))
>  		p[i] =  0x11;
> 
>  	madvise(p, SIZE, MADV_PAGEOUT);
>  }
> 
> w/o patch                    w/ patch
> root@10:~# time ./a.out      root@10:~# time ./a.out
> real	0m49.634s            real   0m46.334s
> user	0m0.637s             user   0m0.648s
> sys	0m47.434s            sys    0m44.265s
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>


Thanks,
SJ

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