Date: Mon, 01 Jul 2024 10:04:39 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: "Huang, Ying" <ying.huang@...el.com>, 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 1 Jul 2024, at 10:03, David Hildenbrand wrote:

> 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? :)).

Sure, let me give it a shot. :)

Best Regards,
Yan, Zi

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