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

["Huang, Ying" <ying.huang@intel.com>]

"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.

> 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.

--
Best Regards,
Huang, Ying