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

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Date: Fri, 21 Jun 2024 22:18:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.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 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.

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.

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?

> 
> --
> Best Regards,
> Yan, Zi

-- 
Cheers,

David / dhildenb