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

[Ryan Roberts]

On 18/01/2024 11:10, Barry Song wrote:
> 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. 

I think this could also violate MADV_NOHUGEPAGE; if the application has
requested that we do not create a THP, then we had better not; it could cause a
correctness issue in some circumstances. You would need to pay attention to this
vma flag if taking this approach.

> 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. 

In fact, even this may not be sufficient; it's possible that a contiguous set of
base pages (small folios) were allocated to a virtual mapping and all swapped
out together - they would likely end up contiguous in the swap file, but should
not be swapped back in as a single folio because of this (same reasoning applies
to cluster of smaller THPs that you mistake for a larger THP, etc).

So you will need to check what THP sizes are enabled and check the VMA
suitability regardless; Perhaps you are already doing this - I haven't looked at
the code yet.

I'll aim to review the code in the next couple of weeks.

Thanks,
Ryan

> 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.
> 
> 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@xxxxxxx/
> [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(-)
>