Re: [PATCH] mm: move idle swap cache pages to the tail of LRU after COW

[Huang, Ying]

Johannes Weiner <hannes@xxxxxxxxxxx> writes:

> On Thu, May 20, 2021 at 09:22:45AM +0800, Huang, Ying wrote:
>> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
>> 
>> > On Wed, May 19, 2021 at 09:33:13AM +0800, Huang Ying wrote:
>> >> diff --git a/mm/memory.c b/mm/memory.c
>> >> index b83f734c4e1d..2b6847f4c03e 100644
>> >> --- a/mm/memory.c
>> >> +++ b/mm/memory.c
>> >> @@ -3012,6 +3012,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>> >>  				munlock_vma_page(old_page);
>> >>  			unlock_page(old_page);
>> >>  		}
>> >> +		if (page_copied && PageSwapCache(old_page) &&
>> >> +		    !page_mapped(old_page) && trylock_page(old_page)) {
>> >> +			try_to_free_idle_swapcache(old_page);
>> >> +			unlock_page(old_page);
>> >
>> > If there are no more swap or pte references, can we just attempt to
>> > free the page right away, like we do during regular unmap?
>> >
>> > 		if (page_copied)
>> > 			free_swap_cache(old_page);
>> > 		put_page(old_page);
>> 
>> A previous version of the patch does roughly this.
>> 
>> https://lore.kernel.org/lkml/20210113024241.179113-1-ying.huang@xxxxxxxxx/
>> 
>> But Linus has concerns with the overhead introduced in the hot COW path.
>
> Sorry, I had missed that thread.
>
> It sounds like there were the same concerns about the LRU shuffling
> overhead in the COW page. Now we have numbers for that, but not the
> free_swap_cache version. Would you be able to run the numbers for that
> as well? It would be interesting to see how much the additional code
> complexity buys us.

The number for which workload?  The workload that is used to evaluate
this patch?

>> Another possibility is to move the idle swap cache page to the tail of
>> the file LRU list.  But the question is how to identify the page.
>
> The LRU type is identified by PG_swapbacked, and we do clear that for
> anon pages to implement MADV_FREE. It may work here too. But I'm
> honestly a bit skeptical about the ROI on this...

The definition of PageSwapCache() is

static __always_inline int PageSwapCache(struct page *page)
{
#ifdef CONFIG_THP_SWAP
	page = compound_head(page);
#endif
	return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags);
}

So we cannot clear PG_swapbacked directly.

Best Regards,
Huang, Ying