Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio isolation + checks under PTL

[Zi Yan <ziy@nvidia.com>]

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On 1 Jul 2024, at 4:32, Huang, Ying wrote:

> "Zi Yan" <ziy@nvidia.com> writes:
>
>> On Wed Jun 26, 2024 at 12:49 PM EDT, David Hildenbrand wrote:
>>> On 21.06.24 22:48, Zi Yan wrote:
>>>> On 21 Jun 2024, at 16:18, David Hildenbrand wrote:
>>>>
>>>>> On 21.06.24 15:44, Zi Yan wrote:
>>>>>> On 20 Jun 2024, at 17:29, David Hildenbrand wrote:
>>>>>>
>>>>>>> Currently we always take a folio reference even if migration will not
>>>>>>> even be tried or isolation failed, requiring us to grab+drop an additional
>>>>>>> reference.
>>>>>>>
>>>>>>> Further, we end up calling folio_likely_mapped_shared() while the folio
>>>>>>> might have already been unmapped, because after we dropped the PTL, that
>>>>>>> can easily happen. We want to stop touching mapcounts and friends from
>>>>>>> such context, and only call folio_likely_mapped_shared() while the folio
>>>>>>> is still mapped: mapcount information is pretty much stale and unreliable
>>>>>>> otherwise.
>>>>>>>
>>>>>>> So let's move checks into numamigrate_isolate_folio(), rename that
>>>>>>> function to migrate_misplaced_folio_prepare(), and call that function
>>>>>>> from callsites where we call migrate_misplaced_folio(), but still with
>>>>>>> the PTL held.
>>>>>>>
>>>>>>> We can now stop taking temporary folio references, and really only take
>>>>>>> a reference if folio isolation succeeded. Doing the
>>>>>>> folio_likely_mapped_shared() + golio isolation under PT lock is now similar
>>>>>>> to how we handle MADV_PAGEOUT.
>>>>>>>
>>>>>>> While at it, combine the folio_is_file_lru() checks.
>>>>>>>
>>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>>> ---
>>>>>>>    include/linux/migrate.h |  7 ++++
>>>>>>>    mm/huge_memory.c        |  8 ++--
>>>>>>>    mm/memory.c             |  9 +++--
>>>>>>>    mm/migrate.c            | 81 +++++++++++++++++++----------------------
>>>>>>>    4 files changed, 55 insertions(+), 50 deletions(-)
>>>>>>
>>>>>> LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>
>>>>>>
>>>>>> One nit below:
>>>>>>
>>>>>> <snip>
>>>>>>
>>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>>> index fc27dabcd8e3..4b2817bb2c7d 100644
>>>>>>> --- a/mm/huge_memory.c
>>>>>>> +++ b/mm/huge_memory.c
>>>>>>> @@ -1688,11 +1688,13 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>>>>>>    	if (node_is_toptier(nid))
>>>>>>>    		last_cpupid = folio_last_cpupid(folio);
>>>>>>>    	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
>>>>>>> -	if (target_nid == NUMA_NO_NODE) {
>>>>>>> -		folio_put(folio);
>>>>>>> +	if (target_nid == NUMA_NO_NODE)
>>>>>>> +		goto out_map;
>>>>>>> +	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
>>>>>>> +		flags |= TNF_MIGRATE_FAIL;
>>>>>>>    		goto out_map;
>>>>>>>    	}
>>>>>>> -
>>>>>>> +	/* The folio is isolated and isolation code holds a folio reference. */
>>>>>>>    	spin_unlock(vmf->ptl);
>>>>>>>    	writable = false;
>>>>>>>
>>>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>>>> index 118660de5bcc..4fd1ecfced4d 100644
>>>>>>> --- a/mm/memory.c
>>>>>>> +++ b/mm/memory.c
>>>>>>
>>>>>> <snip>
>>>>>>
>>>>>>> @@ -5345,10 +5343,13 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>>>>>>    	else
>>>>>>>    		last_cpupid = folio_last_cpupid(folio);
>>>>>>>    	target_nid = numa_migrate_prep(folio, vmf, vmf->address, nid, &flags);
>>>>>>> -	if (target_nid == NUMA_NO_NODE) {
>>>>>>> -		folio_put(folio);
>>>>>>> +	if (target_nid == NUMA_NO_NODE)
>>>>>>> +		goto out_map;
>>>>>>> +	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
>>>>>>> +		flags |= TNF_MIGRATE_FAIL;
>>>>>>>    		goto out_map;
>>>>>>>    	}
>>>>>>
>>>>>> These two locations are repeated code, maybe just merge the ifs into
>>>>>> numa_migrate_prep(). Feel free to ignore if you are not going to send
>>>>>> another version. :)
>>>>>
>>>>> I went back and forth a couple of times and
>>>>>
>>>>> a) Didn't want to move numa_migrate_prep() into
>>>>>     migrate_misplaced_folio_prepare(), because having that code in
>>>>>     mm/migrate.c felt a bit odd.
>>>>
>>>> I agree after checking the actual code, since the code is just
>>>> updating NUMA fault stats and checking where the folio should be.
>>>>
>>>>>
>>>>> b) Didn't want to move migrate_misplaced_folio_prepare() because I enjoy
>>>>>     seeing the migrate_misplaced_folio_prepare() and
>>>>>     migrate_misplaced_folio() calls in the same callercontext.
>>>>>
>>>>> I also considered renaming numa_migrate_prep(), but wasn't really able to come up with a good name.
>>>>
>>>> How about numa_migrate_check()? Since it tells whether a folio should be
>>>> migrated or not.
>>>>
>>>>>
>>>>> But maybe a) is not too bad?
>>>>>
>>>>> We'd have migrate_misplaced_folio_prepare() consume &flags and &target_nid, and perform the "flags |= TNF_MIGRATE_FAIL;" internally.
>>>>>
>>>>> What would be your take?
>>>>
>>>> I would either rename numa_migrate_prep() or just do nothing. I have to admit
>>>> that the "prep" and "prepare" in both function names motivated me to propose
>>>> the merge, but now the actual code tells me they should be separate.
>>>
>>> Let's leave it like that for now. Renaming to numa_migrate_check() makes
>>> sense, and likely moving more numa handling stuff in there.
>>>
>>> Bit I yet have to figure out why some of the memory.c vs. huge_memory.c
>>> code differences exist, so we can unify them.
>>>
>>> For example, why did 33024536bafd9 introduce slightly different
>>> last_cpupid handling in do_huge_pmd_numa_page(), whereby it seems like
>>> some subtle difference in handling NUMA_BALANCING_MEMORY_TIERING? Maybe
>>> I am missing something obvious. :)
>>
>> It seems to me that a sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING
>> check is missing in do_huge_pmd_numa_page(). So the
>>
>> if (node_is_toptier(nid))
>>
>> should be
>>
>> if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) ||
>> node_is_toptier(nid))
>>
>> to be consistent with other checks. Add Ying to confirm.
>
> Yes.  It should be so.  Sorry for my mistake and confusing.

Thank you for the confirmation.

>
>> I also think a function like
>>
>> bool folio_has_cpupid(folio)
>> {
>>     return !(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING)
>>     || node_is_toptier(folio_nid(folio));
>> }
>>
>> would be better than the existing checks.
>
> Yes.  This looks better.  Even better, we can add some comments to the
> function too.

I will prepare a patch about it.

Best Regards,
Yan, Zi

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