Re: [PATCH v1 10/10] mm: Allocate large folios for anonymous memory

[Yu Zhao]

On Tue, Jun 27, 2023 at 3:57 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 27/06/2023 04:01, Yu Zhao wrote:
> > On Mon, Jun 26, 2023 at 11:15 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> With all of the enabler patches in place, modify the anonymous memory
> >> write allocation path so that it opportunistically attempts to allocate
> >> a large folio up to `max_anon_folio_order()` size (This value is
> >> ultimately configured by the architecture). This reduces the number of
> >> page faults, reduces the size of (e.g. LRU) lists, and generally
> >> improves performance by batching what were per-page operations into
> >> per-(large)-folio operations.
> >>
> >> If CONFIG_LARGE_ANON_FOLIO is not enabled (the default) then
> >> `max_anon_folio_order()` always returns 0, meaning we get the existing
> >> allocation behaviour.
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >>  mm/memory.c | 159 +++++++++++++++++++++++++++++++++++++++++++++++-----
> >>  1 file changed, 144 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index a8f7e2b28d7a..d23c44cc5092 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3161,6 +3161,90 @@ static inline int max_anon_folio_order(struct vm_area_struct *vma)
> >>                 return CONFIG_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX;
> >>  }
> >>
> >> +/*
> >> + * Returns index of first pte that is not none, or nr if all are none.
> >> + */
> >> +static inline int check_ptes_none(pte_t *pte, int nr)
> >> +{
> >> +       int i;
> >> +
> >> +       for (i = 0; i < nr; i++) {
> >> +               if (!pte_none(ptep_get(pte++)))
> >> +                       return i;
> >> +       }
> >> +
> >> +       return nr;
> >> +}
> >> +
> >> +static int calc_anon_folio_order_alloc(struct vm_fault *vmf, int order)
> >
> > As suggested previously in 03/10, we can leave this for later.
>
> I disagree. This is the logic that prevents us from accidentally replacing
> already set PTEs, or wandering out of the VMA bounds etc. How would you catch
> all those corener cases without this?

Again, sorry for not being clear previously: we definitely need to
handle alignments & overlapps. But the fallback, i.e., "for (; order >
1; order--) {" in calc_anon_folio_order_alloc() is not necessary.

For now, we just need something like

  bool is_order_suitable() {
    // check whether it fits properly
  }

Later on, we could add

  alloc_anon_folio_best_effort()
  {
    for a list of fallback orders
      is_order_suitable()
  }