Re: [PATCH RFC 0/6] mm: support large folios swap-in

Huang, Ying
2024-01-29T09:07:04Z

Barry Song <21cnbao@gmail.com> writes:

> 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 back-
> ground, 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.
>
> In theory, we don't need to rely on Ryan's swap out patchset[1]. That is to say,
> before swap-out, if some memory were normal pages, but when swapping in, we
> can also swap-in them as large folios. But this might require I/O happen at
> some random places in swap devices. So we limit the large folios swap-in to
> those areas which were large folios before swapping-out, aka, swaps are also
> contiguous in hardware. On the other hand, in OPPO's product, we've deployed
> anon large folios on millions of phones[2]. we enhanced zsmalloc and zRAM to
> compress and decompress large folios as a whole, which help improve compression
> ratio and decrease CPU consumption significantly. In zsmalloc and zRAM we can
> save large objects whose original size are 64KiB for example. So it is also a
> better choice for us to only swap-in large folios for those compressed large
> objects as a large folio can be decompressed all together.

Another possibility is to combine large folios swap-in with VMA based
swap-in readahead. If we will swap-in readahead several pages based on
VMA, we can swap-in a large folio instead.

I think that it is similar as allocating large file folios for file
readahead (TBH, I haven't check file large folios allocation code).

--
Best Regards,
Huang, Ying

> Note I am moving my previous "arm64: mm: swap: support THP_SWAP on hardware
> with MTE" to this series as it might help review.
>
> [1] [PATCH v3 0/4] Swap-out small-sized THP without splitting
> https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.com/
> [2] OnePlusOSS / android_kernel_oneplus_sm8550
> https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/tree/oneplus/sm8550_u_14.0.0_oneplus11
>
> Barry Song (2):
> arm64: mm: swap: support THP_SWAP on hardware with MTE
> mm: rmap: weaken the WARN_ON in __folio_add_anon_rmap()
>
> Chuanhua Han (4):
> mm: swap: introduce swap_nr_free() for batched swap_free()
> mm: swap: make should_try_to_free_swap() support large-folio
> mm: support large folios swapin as a whole
> mm: madvise: don't split mTHP for MADV_PAGEOUT
>
> arch/arm64/include/asm/pgtable.h | 21 ++----
> arch/arm64/mm/mteswap.c | 42 ++++++++++++
> include/asm-generic/tlb.h | 10 +++
> include/linux/huge_mm.h | 12 ----
> include/linux/pgtable.h | 62 ++++++++++++++++-
> include/linux/swap.h | 6 ++
> mm/madvise.c | 48 ++++++++++++++
> mm/memory.c | 110 ++++++++++++++++++++++++++-----
> mm/page_io.c | 2 +-
> mm/rmap.c | 5 +-
> mm/swap_slots.c | 2 +-
> mm/swapfile.c | 29 ++++++++
> 12 files changed, 301 insertions(+), 48 deletions(-)