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

[Barry Song]

On Fri, Mar 15, 2024 at 9:43 PM Huang, Ying <ying.huang at intel.com> wrote:
>
> Barry Song <21cnbao at gmail.com> writes:
>
> > From: Chuanhua Han <hanchuanhua at oppo.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