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 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.

--
Best Regards,
Yan, Zi

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