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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Fri, 21 Jun 2024 09:32:57 +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 04:05, 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(-)
>>
>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>> index f9d92482d117..644be30b69c8 100644
>> --- a/include/linux/migrate.h
>> +++ b/include/linux/migrate.h
>> @@ -139,9 +139,16 @@ const struct movable_operations *page_movable_ops(struct page *page)
>>   }
>>
>>   #ifdef CONFIG_NUMA_BALANCING
>> +int migrate_misplaced_folio_prepare(struct folio *folio,
>> +		struct vm_area_struct *vma, int node);
>>   int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
>>   			   int node);
>>   #else
>> +static inline int migrate_misplaced_folio_prepare(struct folio *folio,
>> +		struct vm_area_struct *vma, int node)
>> +{
>> +	return -EAGAIN; /* can't migrate now */
>> +}
>>   static inline int migrate_misplaced_folio(struct folio *folio,
>>   					 struct vm_area_struct *vma, int node)
>>   {
>> 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
>> @@ -5207,8 +5207,6 @@ int numa_migrate_prep(struct folio *folio, struct vm_fault *vmf,
>>   {
>>   	struct vm_area_struct *vma = vmf->vma;
>>
>> -	folio_get(folio);
>> -
>>   	/* Record the current PID acceesing VMA */
>>   	vma_set_access_pid_bit(vma);
> 
> Without taking a reference here, is it safe to call mpol_misplaced() on the folio
> at the return? Can this folio be freed?

As long as we are holding the PTL, the folio cannot get unmapped and 
consequently not freed.

Note that if that would be the case (being able to get freed 
concurrently), even the folio_get() would have been wrong: if it could 
get freed concurrently, we would have to use folio_try_get*().

Thanks!

-- 
Cheers,

David / dhildenb