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

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Message-ID: <45457815-66c4-029f-42f9-6c377e4eb1e3@intel.com>
Date:   Fri, 4 Aug 2023 08:17:25 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Yu Zhao <yuzhao@...gle.com>
CC:     Ryan Roberts <ryan.roberts@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <willy@...radead.org>,
        <vishal.moola@...il.com>, <wangkefeng.wang@...wei.com>,
        <minchan@...nel.org>, <david@...hat.com>, <shy828301@...il.com>
Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing



On 8/4/2023 7:38 AM, Yu Zhao wrote:
> On Thu, Aug 3, 2023 at 5:27 PM Yin, Fengwei <fengwei.yin@...el.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@...el.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@...r.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.


Regards
Yin, Fengwei