Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

David Hildenbrand
2023-08-04T07:34:30Z

On 04.08.23 02:17, Yin, Fengwei wrote:
>
>
> On 8/4/2023 7:38 AM, Yu Zhao wrote:
>> On Thu, Aug 3, 2023 at 5:27 PM Yin, Fengwei <fengwei.yin@intel.com> wrote:
>>>
>>>
>>>
>>> On 8/4/2023 4:46 AM, Yu Zhao wrote:
>>>> On Wed, Aug 2, 2023 at 6:56 AM Yin, Fengwei <fengwei.yin@intel.com> wrote:
>>>>>
>>>>> "
>>>>>
>>>>> On 8/2/2023 8:49 PM, Ryan Roberts wrote:
>>>>>> On 02/08/2023 13:42, Yin, Fengwei wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 8/2/2023 8:40 PM, Ryan Roberts wrote:
>>>>>>>> On 02/08/2023 13:35, Yin, Fengwei wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 8/2/2023 6:27 PM, Ryan Roberts wrote:
>>>>>>>>>> On 28/07/2023 17:13, Yin Fengwei wrote:
>>>>>>>>>>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
>>>>>>>>>>> folio_mapcount() is used to check whether the folio is shared. But it's
>>>>>>>>>>> not correct as folio_mapcount() returns total mapcount of large folio.
>>>>>>>>>>>
>>>>>>>>>>> Use folio_estimated_sharers() here as the estimated number is enough.
>>>>>>>>>>>
>>>>>>>>>>> Yin Fengwei (2):
>>>>>>>>>>> madvise: don't use mapcount() against large folio for sharing check
>>>>>>>>>>> madvise: don't use mapcount() against large folio for sharing check
>>>>>>>>>>>
>>>>>>>>>>> mm/huge_memory.c | 2 +-
>>>>>>>>>>> mm/madvise.c | 6 +++---
>>>>>>>>>>> 2 files changed, 4 insertions(+), 4 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> As a set of fixes, I agree this is definitely an improvement, so:
>>>>>>>>>>
>>>>>>>>>> Reviewed-By: Ryan Roberts
>>>>>>>>> Thanks.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> But I have a couple of comments around further improvements;
>>>>>>>>>>
>>>>>>>>>> Once we have the scheme that David is working on to be able to provide precise
>>>>>>>>>> exclusive vs shared info, we will probably want to move to that. Although that
>>>>>>>>>> scheme will need access to the mm_struct of a process known to be mapping the
>>>>>>>>>> folio. We have that info, but its not passed to folio_estimated_sharers() so we
>>>>>>>>>> can't just reimplement folio_estimated_sharers() - we will need to rework these
>>>>>>>>>> call sites again.
>>>>>>>>> Yes. This could be extra work. Maybe should delay till David's work is done.
>>>>>>>>
>>>>>>>> What you have is definitely an improvement over what was there before. And is
>>>>>>>> probably the best we can do without David's scheme. So I wouldn't delay this.
>>>>>>>> Just pointing out that we will be able to make it even better later on (if
>>>>>>>> David's stuff goes in).
>>>>>>> Yes. I agree that we should wait for David's work ready and do fix based on that.
>>>>>>
>>>>>> I was suggesting the opposite - not waiting. Then we can do separate improvement
>>>>>> later.
>>>>> Let's wait for David's work ready.
>>>>
>>>> Waiting is fine as long as we don't miss the next merge window -- we
>>>> don't want these two bugs to get into another release. Also I think we
>>>> should cc stable, since as David mentioned, they have been causing
>>>> selftest failures.
>>>
>>> Stable was CCed.
>>
>> Need to add the "Cc: stable@vger.kernel.org" tag:
>> Documentation/process/stable-kernel-rules.rst
> OK. Thanks for clarification. I totally mis-understanded this. :).
>
> I'd like to wait for answer from Andrew whether these patches are suitable
> for stable (I suppose you think so) branch.

Note that the COW test does not fail -- it skips -- but the behavir changed:

$ ./cow
# [INFO] detected THP size: 2048 KiB
# [INFO] detected hugetlb page size: 2048 KiB
# [INFO] detected hugetlb page size: 1048576 KiB
# [INFO] huge zeropage is enabled
TAP version 13
1..190
# [INFO] Anonymous memory tests in private mappings
# [RUN] Basic COW after fork() ... with base page
ok 1 No leak from parent into child
# [RUN] Basic COW after fork() ... with swapped out base page
ok 2 No leak from parent into child
# [RUN] Basic COW after fork() ... with THP
ok 3 No leak from parent into child
# [RUN] Basic COW after fork() ... with swapped-out THP
ok 4 No leak from parent into child
# [RUN] Basic COW after fork() ... with PTE-mapped THP
ok 5 No leak from parent into child
# [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP
ok 6 # SKIP MADV_PAGEOUT did not work, is swap enabled?
# [RUN] Basic COW after fork() ... with single PTE of THP
ok 7 No leak from parent into child
# [RUN] Basic COW after fork() ... with single PTE of swapped-out THP
ok 8 No leak from parent into child
# [RUN] Basic COW after fork() ... with partially mremap()'ed THP
ok 9 No leak from parent into child
# [RUN] Basic COW after fork() ... with partially shared THP
ok 10 No leak from parent into child
...

Observe how patch #6 skips because the MADV_PAGEOUT was not effective (which might have happened due to other reasons as well, thus no failure).

The code that broke it is

commit 07e8c82b5eff8ef34b74210eacb8d9c4a2886b82
Author: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Date: Wed Dec 21 10:08:46 2022 -0800

madvise: convert madvise_cold_or_pageout_pte_range() to use folios

This change removes a number of calls to compound_head(), and saves
1729 bytes of kernel text.

Link: https://lkml.kernel.org/r/20221221180848.20774-3-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Ever since v6.3.

The simplest way to fix it would be to revert the page_mapcount() -> folio_mapcount(),
conversion.

Probably all that is information worth having in the patch description.

--
Cheers,

David / dhildenb