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(-) >
