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

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> On Fri, Mar 15, 2024 at 9:43 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> 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.
>
> I'd rather try the reverse direction as non-sync anon memory is only around
> 3% in a phone as my observation.

Phone is not the only platform that Linux is running on.

>>
>> 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.

I don't think that we have a sophisticated policy to allocate large
folio.  Large folio could waste memory for some workloads, so I think
that it's a good idea to allocate large folio always.

Readahead gives us an opportunity to play with the policy.

> 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.

I am not against to support large folio for compressing/decompressing.

I just suggest to do that later, after we play with normal swap-in.
SWAP_SYCHRONOUS related swap-in code is an optimization based on normal
swap.  So, it seems natural to support large folio swap-in for normal
swap-in firstly.

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

We can take advantage of readahead algorithm in SWAP_SYCHRONOUS
optimization too.  The sub-pages that is not accessed by page fault can
be treated as readahead.  I think that is a better policy than
allocating large folio always.

>>
>> 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