linux-kernel - Re: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: fix memory loads ordering

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Message-ID: <25e02685-4f1d-47fa-be5b-01ff85bb0ce2@redhat.com>
Date: Fri, 10 Jan 2025 20:17:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Yu Zhao <yuzhao@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Mateusz Guzik <mjguzik@...il.com>, Muchun Song <muchun.song@...ux.dev>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Will Deacon <will@...nel.org>
Subject: Re: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: fix memory loads
 ordering

On 10.01.25 20:04, David Hildenbrand wrote:
> On 07.01.25 18:02, David Hildenbrand wrote:
>> On 07.01.25 17:35, Matthew Wilcox wrote:
>>> On Tue, Jan 07, 2025 at 09:49:18AM +0100, David Hildenbrand wrote:
>>>>> +++ b/include/linux/page-flags.h
>>>>> @@ -212,7 +212,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
>>>>>      	 * cold cacheline in some cases.
>>>>>      	 */
>>>>>      	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
>>>>> -	    test_bit(PG_head, &page->flags)) {
>>>>> +	    test_bit_acquire(PG_head, &page->flags)) {
>>>>
>>>> This change will affect all page_fixed_fake_head() users, like ordinary
>>>> PageTail even on !hugetlb.
>>>
>>> I've been looking at the callers of PageTail() because it's going to
>>> be a bit of a weird thing to be checking in the separate-page-and-folio
>>> world.  Obviously we can implement it, but there's a bit of a "But why
>>> would you want to ask that question" question.
>>>
>>> Most current occurrences of PageTail() are in assertions of one form or
>>> another.  Fair enough, not performance critical.
>>>
>>> make_device_exclusive_range() is a little weird; looks like it's trying
>>> to make sure that each folio is only made exclusive once, and ignore any
>>> partial folios which overlap the start of the area.
>>
>> I could have sworn we only support small folios here, but looks like
>> we do support large folios.
>>
>> IIUC, there is no way to identify reliably "this folio is device exclusive",
>> the only hint is "no mappings". The following might do:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index c6c4d4ea29a7e..1424d0a351a86 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2543,7 +2543,13 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
>>     
>>            for (i = 0; i < npages; i++, start += PAGE_SIZE) {
>>                    struct folio *folio = page_folio(pages[i]);
>> -               if (PageTail(pages[i]) || !folio_trylock(folio)) {
>> +
>> +               /*
>> +                * If there are no mappings, either the folio is actually
>> +                * unmapped or only device-exclusive swap entries point at
>> +                * this folio.
>> +                */
>> +               if (!folio_mapped(folio) || !folio_trylock(folio)) {
>>                            folio_put(folio);
>>                            pages[i] = NULL;
>>                            continue;
> 
> I stared longer at this, and not sure if that will work.
> 
> The PageTail() is in place because we return with the folio locked on
> success, so we won't trylock again on tail pages.
> 
> But staring at page_make_device_exclusive_one(), I am not sure if it
> does what we want in all cases ...
> 
> ... and the hmm selftests just keeps failing upstream as well?! huh. :)
> 
> I'll try spending some time on this to see if I can grasp what needs to
> be done and how it could be handled ... better.
> 

As expected ...

# echo never > /sys/kernel/mm/transparent_hugepage/enabled
# ./hmm-tests
...
#  RUN           hmm.hmm_device_private.exclusive ...
#            OK  hmm.hmm_device_private.exclusive
ok 21 hmm.hmm_device_private.exclusive
#  RUN           hmm.hmm_device_private.exclusive_mprotect ...
#            OK  hmm.hmm_device_private.exclusive_mprotect
ok 22 hmm.hmm_device_private.exclusive_mprotect
#  RUN           hmm.hmm_device_private.exclusive_cow ...
#            OK  hmm.hmm_device_private.exclusive_cow
ok 23 hmm.hmm_device_private.exclusive_cow
#  RUN           hmm.hmm_device_private.hmm_gup_test ...
#            OK  hmm.hmm_device_private.hmm_gup_test
...

# echo always > /sys/kernel/mm/transparent_hugepage/enabled
...
#  RUN           hmm.hmm_device_private.exclusive ...
# hmm-tests.c:1751:exclusive:Expected ret (-16) == 0 (0)
# exclusive: Test terminated by assertion
#          FAIL  hmm.hmm_device_private.exclusive
not ok 21 hmm.hmm_device_private.exclusive
#  RUN           hmm.hmm_device_private.exclusive_mprotect ...
# hmm-tests.c:1805:exclusive_mprotect:Expected ret (-16) == 0 (0)
# exclusive_mprotect: Test terminated by assertion
#          FAIL  hmm.hmm_device_private.exclusive_mprotect
not ok 22 hmm.hmm_device_private.exclusive_mprotect
#  RUN           hmm.hmm_device_private.exclusive_cow ...
# hmm-tests.c:1858:exclusive_cow:Expected ret (-16) == 0 (0)
# exclusive_cow: Test terminated by assertion
#          FAIL  hmm.hmm_device_private.exclusive_cow
not ok 23 hmm.hmm_device_private.exclusive_cow


So rejecting folio_test_large() would likely achieve the same thing 
right now.

-- 
Cheers,

David / dhildenb