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 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.

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.

-- 
Best Regards,
Yan, Zi


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