Re: [PATCH v2 3/3] madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check

[Yin, Fengwei]

On 8/16/2023 7:44 PM, Daniel Gomez wrote:
> On Wed, Aug 16, 2023 at 07:30:35AM +0800, Yin Fengwei wrote:
>>
>>
>> On 8/15/23 21:25, Daniel Gomez wrote:
>>> Hi Yin,
>>> On Tue, Aug 08, 2023 at 10:09:17AM +0800, Yin Fengwei wrote:
>>>> Commit 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a
>>>> folio") replaced the page_mapcount() with folio_mapcount() to check
>>>> whether the folio is shared by other mapping.
>>>>
>>>> It's not correct for large folios. folio_mapcount() returns the total
>>>> mapcount of large folio which is not suitable to detect whether the folio
>>>> is shared.
>>>>
>>>> Use folio_estimated_sharers() which returns a estimated number of shares.
>>>> That means it's not 100% correct. It should be OK for madvise case here.
>>>
>>> I'm trying to understand why it should be ok for madvise this change, so
>>> I hope it's okay to ask you few questions.
>>>
>>> folio_mapcount() calculates the total maps for all the subpages of a
>>> folio. However, the folio_estimated_sharers does it only for the first
>>> subpage making it not true for large folios. Then, wouldn't this change
>>> drop support for large folios?
>> I saw David explained this very well in another mail.
>>
>>>
>>> Seems like folio_entire_mapcount() is not accurate either because of it
>>> does not inclue PTE-mapped sub-pages which I think we need here. Hence,
>>> the folio_mapcount(). Could this be something missing in the test side?
>>>
>>> I tried to replicate the setup with CONFIG_TRANSPARENT_HUGEPAGE but
>>> seems like I'm not able to do it:
>>>
>>> ./cow
>>> # [INFO] detected THP size: 2048 KiB
>>> # [INFO] detected hugetlb size: 2048 KiB
>>> # [INFO] detected hugetlb size: 1048576 KiB
>>> # [INFO] huge zeropage is enabled
>>> TAP version 13
>>> 1..166
>>> # [INFO] Anonymous memory tests in private mappings
>>> # [RUN] Basic COW after fork() ... with base page
>>> not ok 1 MADV_NOHUGEPAGE failed
>>> # [RUN] Basic COW after fork() ... with swapped out base page
>>> not ok 2 MADV_NOHUGEPAGE failed
>>> # [RUN] Basic COW after fork() ... with THP
>>> not ok 3 MADV_HUGEPAGE failed
>>> # [RUN] Basic COW after fork() ... with swapped-out THP
>>> not ok 4 MADV_HUGEPAGE failed
>>> # [RUN] Basic COW after fork() ... with PTE-mapped THP
>>> not ok 5 MADV_HUGEPAGE failed
>>> # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP
>>> not ok 6 MADV_HUGEPAGE failed
>>> ...
>> Can you post the MADV_PAGEOUT and PTE-mapped THP related testing result?
>> And I suppose swap need be enabled also for the testing.
> 
> You may find a dump of the logs in the link below with system information. Let me
> know if you find something wrong in my setup or if you need something else.
> Besides CONFIG_TRANSPARENT_HUGEPAGE, CONFIG_SWAP is also enabled in the kernel.
> 
> https://gitlab.com/-/snippets/2584135
> 
> Also, strace reports ENOSYS for MADV_*:
> madvise(0x7f2912465000, 4096, MADV_NOHUGEPAGE) = -1 ENOSYS (Function not implemented)
> madvise(0x7f2912000000, 2097152, MADV_HUGEPAGE) = -1 ENOSYS (Function not implemented)
O. The problem here is MADV_HUGEPAGE/MADV_NOHUGEPAGE doesn't work.
Do you have CONFIG_ADVISE_SYSCALLS enabled?


Regards
Yin, Fengwei

> 
> 
>>
>>
>> Regards
>> Yin, Fengwei
>>
>>>
>>>
>>> Daniel
>>>>
>>>> User-visible effects is that the THP is skipped when user call madvise.
>>>> But the correct behavior is THP should be split and processed then.
>>>>
>>>> NOTE: this change is a temporary fix to reduce the user-visible effects
>>>> before the long term fix from David is ready.
>>>>
>>>> Fixes: 98b211d6415f ("madvise: convert madvise_free_pte_range() to use a folio")
>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>>>> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>>>> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>> ---
>>>>  mm/madvise.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>>> index 49af35e2d99a..4dded5d27e7e 100644
>>>> --- a/mm/madvise.c
>>>> +++ b/mm/madvise.c
>>>> @@ -683,7 +683,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>>  		if (folio_test_large(folio)) {
>>>>  			int err;
>>>>
>>>> -			if (folio_mapcount(folio) != 1)
>>>> +			if (folio_estimated_sharers(folio) != 1)
>>>>  				break;
>>>>  			if (!folio_trylock(folio))
>>>>  				break;
>>>> --
>>>> 2.39.2
>>> >