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

[Barry Song <21cnbao@gmail.com>]

On Thu, Jan 18, 2024 at 11:25 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> 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.

we are actually re-using your alloc_anon_folio() by adding a parameter
to make it
support both do_anon_page and do_swap_page,

-static struct folio *alloc_anon_folio(struct vm_fault *vmf)
+static struct folio *alloc_anon_folio(struct vm_fault *vmf,
+      bool (*pte_range_check)(pte_t *, int))
 {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
  struct vm_area_struct *vma = vmf->vma;
@@ -4190,7 +4270,7 @@ static struct folio *alloc_anon_folio(struct
vm_fault *vmf)
  order = highest_order(orders);
  while (orders) {
  addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
- if (pte_range_none(pte + pte_index(addr), 1 << order))
+ if (pte_range_check(pte + pte_index(addr), 1 << order))
  break;
  order = next_order(&orders, order);
  }
@@ -4269,7 +4349,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
  if (unlikely(anon_vma_prepare(vma)))
  goto oom;
  /* Returns NULL on OOM or ERR_PTR(-EAGAIN) if we must retry the fault */
- folio = alloc_anon_folio(vmf);
+ folio = alloc_anon_folio(vmf, pte_range_none);
  if (IS_ERR(folio))
  return 0;
  if (!folio)
--

I assume this has checked everything?

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

nice, thanks!

>
> 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@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(-)
> >
>

Thanks
Barry