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

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>
> 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.

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.

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