On Mon, Jun 26, 2023 at 11:15 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > For variable-order anonymous folios, we need to determine the order that > we will allocate. From a SW perspective, the higher the order we > allocate, the less overhead we will have; fewer faults, fewer folios in > lists, etc. But of course there will also be more memory wastage as the > order increases. > > From a HW perspective, there are memory block sizes that can be > beneficial to reducing TLB pressure. arm64, for example, has the ability > to map "contpte" sized chunks (64K for a 4K base page, 2M for 16K and > 64K base pages) such that one of these chunks only uses a single TLB > entry. > > So we let the architecture specify the order of the maximally beneficial > mapping unit when PTE-mapped. Furthermore, because in some cases, this > order may be quite big (and therefore potentially wasteful of memory), > allow the arch to specify 2 values; One is the max order for a mapping > that _would not_ use THP if all size and alignment constraints were met, > and the other is the max order for a mapping that _would_ use THP if all > those constraints were met. > > Implement this with Kconfig by introducing some new options to allow the > architecture to declare that it supports large anonymous folios along > with these 2 preferred max order values. Then introduce a user-facing > option, LARGE_ANON_FOLIO, which defaults to disabled and can only be > enabled if the architecture has declared its support. When disabled, it > forces the max order values, LARGE_ANON_FOLIO_NOTHP_ORDER_MAX and > LARGE_ANON_FOLIO_THP_ORDER_MAX to 0, meaning only a single page is ever > allocated. > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> > --- > mm/Kconfig | 39 +++++++++++++++++++++++++++++++++++++++ > mm/memory.c | 8 ++++++++ > 2 files changed, 47 insertions(+) > > diff --git a/mm/Kconfig b/mm/Kconfig > index 7672a22647b4..f4ba48c37b75 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1208,4 +1208,43 @@ config PER_VMA_LOCK > > source "mm/damon/Kconfig" > > +config ARCH_SUPPORTS_LARGE_ANON_FOLIO > + def_bool n > + help > + An arch should select this symbol if wants to allow LARGE_ANON_FOLIO > + to be enabled. It must also set the following integer values: > + - ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX > + - ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX > + > +config ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX > + int > + help > + The maximum size of folio to allocate for an anonymous VMA PTE-mapping > + that does not have the MADV_HUGEPAGE hint set. > + > +config ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX > + int > + help > + The maximum size of folio to allocate for an anonymous VMA PTE-mapping > + that has the MADV_HUGEPAGE hint set. > + > +config LARGE_ANON_FOLIO > + bool "Allocate large folios for anonymous memory" > + depends on ARCH_SUPPORTS_LARGE_ANON_FOLIO > + default n > + help > + Use large (bigger than order-0) folios to back anonymous memory where > + possible. This reduces the number of page faults, as well as other > + per-page overheads to improve performance for many workloads. > + > +config LARGE_ANON_FOLIO_NOTHP_ORDER_MAX > + int > + default 0 if !LARGE_ANON_FOLIO > + default ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX > + > +config LARGE_ANON_FOLIO_THP_ORDER_MAX > + int > + default 0 if !LARGE_ANON_FOLIO > + default ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX > + > endmenu I don't think an MVP should add this many Kconfigs. One Kconfig sounds reasonable to me for now.
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