Re: [PATCH v2] mm: madvise: pageout: ignore references rather than clearing young

[Minchan Kim]

On Mon, Feb 26, 2024 at 01:57:39PM +1300, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
> 
> 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 (though the time gap is actually quite small since
> checking PTEs is done immediately after clearing PTEs young). But
> with this 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. Or ignoring the
> new access during the quite small time gap is even better.
> 
> For DAMON's DAMOS_PAGEOUT based on physical address region, we
> still keep its behaviour as is since a physical address might
> be mapped by multiple processes. MADV_PAGEOUT based on virtual
> address is actually much more aggressive on reclamation. To
> untouch paddr's DAMOS_PAGEOUT, we simply pass ignore_references
> as false in reclaim_pages().
> 
> A microbench as below has shown 6% decrement on the latency of
> MADV_PAGEOUT,
> 
>  #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
> 
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>

Acked-by: Minchan Kim <minchan@xxxxxxxxxx>