linux-kernel - Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

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Date: Fri, 15 Mar 2024 21:54:29 +1300
From: Barry Song <21cnbao@...il.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	ryan.roberts@....com, chengming.zhou@...ux.dev, chrisl@...nel.org, 
	david@...hat.com, hannes@...xchg.org, kasong@...cent.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	mhocko@...e.com, nphamcs@...il.com, shy828301@...il.com, steven.price@....com, 
	surenb@...gle.com, wangkefeng.wang@...wei.com, xiang@...nel.org, 
	yosryahmed@...gle.com, yuzhao@...gle.com, Chuanhua Han <hanchuanhua@...o.com>, 
	Barry Song <v-songbaohua@...o.com>
Subject: Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

On Fri, Mar 15, 2024 at 9:43 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Barry Song <21cnbao@...il.com> writes:
>
> > From: Chuanhua Han <hanchuanhua@...o.com>
> >
> > On an embedded system like Android, more than half of anon memory is
> > actually in swap devices such as zRAM. For example, while an app is
> > switched to background, its most memory might be swapped-out.
> >
> > Now we have mTHP features, unfortunately, if we don't support large folios
> > swap-in, once those large folios are swapped-out, we immediately lose the
> > performance gain we can get through large folios and hardware optimization
> > such as CONT-PTE.
> >
> > This patch brings up mTHP swap-in support. Right now, we limit mTHP swap-in
> > to those contiguous swaps which were likely swapped out from mTHP as a
> > whole.
> >
> > Meanwhile, the current implementation only covers the SWAP_SYCHRONOUS
> > case. It doesn't support swapin_readahead as large folios yet since this
> > kind of shared memory is much less than memory mapped by single process.
>
> In contrast, I still think that it's better to start with normal swap-in
> path, then expand to SWAP_SYCHRONOUS case.

I'd rather try the reverse direction as non-sync anon memory is only around
3% in a phone as my observation.

>
> In normal swap-in path, we can take advantage of swap readahead
> information to determine the swapped-in large folio order.  That is, if
> the return value of swapin_nr_pages() > 1, then we can try to allocate
> and swapin a large folio.

I am not quite sure we still need to depend on this. in do_anon_page,
we have broken the assumption and allocated a large folio directly.

On the other hand, compressing/decompressing large folios as a
whole rather than doing it one by one can save a large percent of
CPUs and provide a much lower compression ratio.  With a hardware
accelerator, this is even faster.

So I'd rather more aggressively get large folios swap-in involved
than depending on readahead.

>
> To do that, we need to track whether the sub-pages are accessed.  I
> guess we need that information for large file folio readahead too.
>
> Hi, Matthew,
>
> Can you help us on tracking whether the sub-pages of a readahead large
> folio has been accessed?
>
> > Right now, we are re-faulting large folios which are still in swapcache as a
> > whole, this can effectively decrease extra loops and early-exitings which we
> > have increased in arch_swap_restore() while supporting MTE restore for folios
> > rather than page. On the other hand, it can also decrease do_swap_page as
> > PTEs used to be set one by one even we hit a large folio in swapcache.
> >
>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry