Re: [PATCH v8 0/7] mm: folio_zero_user: clear contiguous pages

Ankur Arora
2025-10-28T17:22:52Z

[ My earlier reply to this ate up some of the headers and broke out of
the thread. Resending. ]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 27 Oct 2025 13:21:02 -0700 Ankur Arora <ankur.a.arora@oracle.com> wrote:
>
>> This series adds clearing of contiguous page ranges for hugepages,
>> improving on the current page-at-a-time approach in two ways:
>>
>> - amortizes the per-page setup cost over a larger extent
>> - when using string instructions, exposes the real region size
>> to the processor.
>>
>> A processor could use a knowledge of the extent to optimize the
>> clearing. AMD Zen uarchs, as an example, elide allocation of
>> cachelines for regions larger than L3-size.
>>
>> Demand faulting a 64GB region shows performance improvements:
>>
>> $ perf bench mem map -p $pg-sz -f demand -s 64GB -l 5
>>
>> baseline +series change
>>
>> (GB/s +- %stdev) (GB/s +- %stdev)
>>
>> pg-sz=2MB 12.92 +- 2.55% 17.03 +- 0.70% + 31.8% preempt=*
>>
>> pg-sz=1GB 17.14 +- 2.27% 18.04 +- 1.05% [#] + 5.2% preempt=none|voluntary
>> pg-sz=1GB 17.26 +- 1.24% 42.17 +- 4.21% +144.3% preempt=full|lazy
>>
>> [#] Milan uses a threshold of LLC-size (~32MB) for eliding cacheline
>> allocation, which is higher than the maximum extent used on x86
>> (ARCH_CONTIG_PAGE_NR=8MB), so preempt=none|voluntary sees no improvement
>> with pg-sz=1GB.
>
> I wasn't understanding this preemption thing at all, but then I saw this
> in the v4 series changelogging:
>
> : [#] Only with preempt=full|lazy because cooperatively preempted models
> : need regular invocations of cond_resched(). This limits the extent
> : sizes that can be cleared as a unit.
>
> Please put this back in!!

/me facepalms. Sorry you had to go that far back.
Yeah, that doesn't make any kind of sense standalone. Will fix.

> It's possible that we're being excessively aggressive with those
> cond_resched()s. Have you investigating tuning their frequency so we
> can use larger extent sizes with these preemption models?

folio_zero_user() does a small part of that: for 2MB pages the clearing
is split in three parts with an intervening cond_resched() for each.

This is of course much simpler than the process_huge_page() approach where
we do a left right dance around the faulting page.

I had implemented a version of process_huge_page() with larger extent
sizes that narrowed as we got closer to the faulting page in [a] (x86
performance was similar to the current series. See [b]).

In hindsight however, that felt too elaborate and probably unnecessary
on most modern systems where you have reasonably large caches.
Where it might help, however, is on more cache constrained systems where
the spatial locality really does matter.

So, my idea was to start with a simple version, get some testing and
then fill in the gaps instead of starting with something like [a].

[a] https://lore.kernel.org/lkml/20220606203725.1313715-1-ankur.a.arora@oracle.com/#r
[b] https://lore.kernel.org/lkml/20220606202109.1306034-1-ankur.a.arora@oracle.com/

>> The anon-w-seq test in the vm-scalability benchmark, however, does show
>> worse performance with utime increasing by ~9%:
>>
>> stime utime
>>
>> baseline 1654.63 ( +- 3.84% ) 811.00 ( +- 3.84% )
>> +series 1630.32 ( +- 2.73% ) 886.37 ( +- 5.19% )
>>
>> 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 a likely uncommon pattern where the memory bandwidth
>> is saturated while also being cache limited because we access the
>> entire region.
>>
>> Raghavendra also tested previous version of the series on AMD Genoa [1].
>
> I suggest you paste Raghavendra's results into this [0/N] - it's
> important material.

Thanks. Will do.

>>
>> ...
>>
>> arch/alpha/include/asm/page.h | 1 -
>> arch/arc/include/asm/page.h | 2 +
>> arch/arm/include/asm/page-nommu.h | 1 -
>> arch/arm64/include/asm/page.h | 1 -
>> arch/csky/abiv1/inc/abi/page.h | 1 +
>> arch/csky/abiv2/inc/abi/page.h | 7 ---
>> arch/hexagon/include/asm/page.h | 1 -
>> arch/loongarch/include/asm/page.h | 1 -
>> arch/m68k/include/asm/page_mm.h | 1 +
>> arch/m68k/include/asm/page_no.h | 1 -
>> arch/microblaze/include/asm/page.h | 1 -
>> arch/mips/include/asm/page.h | 1 +
>> arch/nios2/include/asm/page.h | 1 +
>> arch/openrisc/include/asm/page.h | 1 -
>> arch/parisc/include/asm/page.h | 1 -
>> arch/powerpc/include/asm/page.h | 1 +
>> arch/riscv/include/asm/page.h | 1 -
>> arch/s390/include/asm/page.h | 1 -
>> arch/sparc/include/asm/page_32.h | 2 +
>> arch/sparc/include/asm/page_64.h | 1 +
>> arch/um/include/asm/page.h | 1 -
>> arch/x86/include/asm/page.h | 6 ---
>> arch/x86/include/asm/page_32.h | 6 +++
>> arch/x86/include/asm/page_64.h | 64 ++++++++++++++++++-----
>> arch/x86/lib/clear_page_64.S | 39 +++-----------
>> arch/xtensa/include/asm/page.h | 1 -
>> include/linux/highmem.h | 29 +++++++++++
>> include/linux/mm.h | 69 +++++++++++++++++++++++++
>> mm/memory.c | 82 ++++++++++++++++++++++--------
>> mm/util.c | 13 +++++
>> 30 files changed, 247 insertions(+), 91 deletions(-)
>
> I guess this is an mm.git thing, with x86 acks (please).

Ack that.

> The documented review activity is rather thin at this time so I'll sit
> this out for a while. Please ping me next week and we can reassess,

Will do. And, thanks for the quick look!

--
ankur