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

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

On Fri, Jan 19, 2024 at 9:25 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 18/01/2024 23:54, Barry Song wrote:
> > 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?
>
> Ahh yes, very good. In that case you can disregard what I said; its already covered.
>
> I notice that this series appears as a reply to my series. I'm not sure what the
> normal convention is, but I expect more people would see it if you posted it as
> its own thread?

yes. i was replying your series because we were using your series to
swapout large folios
without splitting. in v2,  i will send it as a new thread.

>
>
> >
> >>
> >> 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
>