Re: [PATCH v10 8/8] mm: folio_zero_user: cache neighbouring pages

[David Hildenbrand (Red Hat)]

On 12/18/25 22:23, Ankur Arora wrote:
> Ankur Arora <ankur.a.arora@xxxxxxxxxx> writes:
>
> > David Hildenbrand (Red Hat) <david@xxxxxxxxxx> writes:
> >
> > > On 12/15/25 21:49, Ankur Arora wrote:
> > > > folio_zero_user() does straight zeroing without caring about
> > > > temporal locality for caches.
> > > > This replaced commit c6ddfb6c5890 ("mm, clear_huge_page: move order
> > > > algorithm into a separate function") where we cleared a page at a
> > > > time converging to the faulting page from the left and the right.
> > > > To retain limited temporal locality, split the clearing in three
> > > > parts: the faulting page and its immediate neighbourhood, and, the
> > > > remaining regions on the left and the right. The local neighbourhood
> > > > will be cleared last.
> > > > Do this only when zeroing small folios (< MAX_ORDER_NR_PAGES) since
> > > > there isn't much expectation of cache locality for large folios.
> > > > Performance
> > > > ===
> > > > AMD Genoa (EPYC 9J14, cpus=2 sockets * 96 cores * 2 threads,
> > > >     memory=2.2 TB, L1d= 16K/thread, L2=512K/thread, L3=2MB/thread)
> > > > anon-w-seq (vm-scalability):
> > > >                               stime                  utime
> > > >     page-at-a-time      1654.63 ( +- 3.84% )     811.00 ( +- 3.84% )
> > > >     contiguous clearing 1602.86 ( +- 3.00% )     970.75 ( +- 4.68% )
> > > >     neighbourhood-last  1630.32 ( +- 2.73% )     886.37 ( +- 5.19% )
> > > > Both stime and utime respond in expected ways. stime drops for both
> > > > contiguous clearing (-3.14%) and neighbourhood-last (-1.46%)
> > > > approaches. However, utime increases for both contiguous clearing
> > > > (+19.7%) and neighbourhood-last (+9.28%).
> > > > In part this is because anon-w-seq runs with 384 processes zeroing
> > > > anonymously mapped memory which they then access sequentially. As
> > > > such this is likely an uncommon pattern where the memory bandwidth
> > > > is saturated while also being cache limited because we access the
> > > > entire region.
> > > > Kernel make workload (make -j 12 bzImage):
> > > >                               stime                  utime
> > > >     page-at-a-time       138.16 ( +- 0.31% )    1015.11 ( +- 0.05% )
> > > >     contiguous clearing  133.42 ( +- 0.90% )    1013.49 ( +- 0.05% )
> > > >     neighbourhood-last   131.20 ( +- 0.76% )    1011.36 ( +- 0.07% )
> > > > For make the utime stays relatively flat with an up to 4.9% improvement
> > > > in the stime.
> > >
> > > Nice evaluation!
> > >
> > > > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> > > > Reviewed-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
> > > > Tested-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
> > > > ---
> > > >    mm/memory.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
> > > >    1 file changed, 42 insertions(+), 2 deletions(-)
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 974c48db6089..d22348b95227 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -7268,13 +7268,53 @@ static void clear_contig_highpages(struct page *page, unsigned long addr,
> > > >     * @addr_hint: The address accessed by the user or the base address.
> > > >     *
> > > >     * Uses architectural support to clear page ranges.
> > > > + *
> > > > + * Clearing of small folios (< MAX_ORDER_NR_PAGES) is split in three parts:
> > > > + * pages in the immediate locality of the faulting page, and its left, right
> > > > + * regions; the local neighbourhood is cleared last in order to keep cache
> > > > + * lines of the faulting region hot.
> > > > + *
> > > > + * For larger folios we assume that there is no expectation of cache locality
> > > > + * and just do a straight zero.
> > >
> > > Just wondering: why not do the same thing here as well? Probably shouldn't hurt
> > > and would get rid of some code?
> >
> > That's a good point. With only a three way split, there's no reason to
> > treat large folios specially.
>
> A bit more on this: this change makes sense but I'll retain the current
> split between patches-7, 8.
>
> Where patch-7, is used to justify using contiguous clearing (and the
> choice of value for PROCESS_PAGES_NON_PREEMPT_BATCH), unit based on
> preemption model etc and patch-8, for the neighbourhood optimization.
>
> > > >     */
> > > >    void folio_zero_user(struct folio *folio, unsigned long addr_hint)
> > > >    {
> > > >    	unsigned long base_addr = ALIGN_DOWN(addr_hint, folio_size(folio));
> > >
> > > While at it you could turn that const as well.
> >
> > Ack.
> >
> > > > +	const long fault_idx = (addr_hint - base_addr) / PAGE_SIZE;
> > > > +	const struct range pg = DEFINE_RANGE(0, folio_nr_pages(folio) - 1);
> > > > +	const int width = 2; /* number of pages cleared last on either side */
> > >
> > > Is "width" really the right terminology? (the way you describe it, it's more
> > > like diameter?)
> >
> > I like diameter. Will make that a define.
>
> I'll make that radius since that's how I'm using it.

All makes sense to me.

--
Cheers

David