Date: Mon, 1 Jul 2024 16:03:00 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, "Huang, Ying" <ying.huang@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio
 isolation + checks under PTL

On 01.07.24 15:50, Zi Yan wrote:
> On 1 Jul 2024, at 4:32, Huang, Ying wrote:
> 
>> "Zi Yan" <ziy@...dia.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@...hat.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@...dia.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.

Do you have capacity to further consolidate the logic, maybe moving more 
stuff into the numa_migrate_prep (and renaming it? :)).

-- 
Cheers,

David / dhildenb

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