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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/18/25 22:23, Ankur Arora wrote:
> 
> Ankur Arora <ankur.a.arora@oracle.com> writes:
> 
>> David Hildenbrand (Red Hat) <david@kernel.org> 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@oracle.com>
>>>> Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>
>>>> Tested-by: Raghavendra K T <raghavendra.kt@amd.com>
>>>> ---
>>>>    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