linux-kernel - Re: [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge

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Date:	Mon, 20 Jun 2016 08:48:17 -0700
From:	"Huang\, Ying" <ying.huang@...el.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	"Huang\, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Jerome Marchand" <jmarchan@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Ebru Akagunduz" <ebru.akagunduz@...il.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge_pmd return value after splitting

Minchan Kim <minchan@...nel.org> writes:

> On Fri, Jun 17, 2016 at 12:45:43PM -0700, Huang, Ying wrote:
>> Minchan Kim <minchan@...nel.org> writes:
>> 
>> > Hi,
>> >
>> > On Thu, Jun 16, 2016 at 08:03:54PM -0700, Huang, Ying wrote:
>> >> From: Huang Ying <ying.huang@...el.com>
>> >> 
>> >> madvise_free_huge_pmd should return 0 if the fallback PTE operations are
>> >> required.  In madvise_free_huge_pmd, if part pages of THP are discarded,
>> >> the THP will be split and fallback PTE operations should be used if
>> >> splitting succeeds.  But the original code will make fallback PTE
>> >> operations skipped, after splitting succeeds.  Fix that via make
>> >> madvise_free_huge_pmd return 0 after splitting successfully, so that the
>> >> fallback PTE operations will be done.
>> >
>> > You're right. Thanks!
>> >
>> >> 
>> >> Know issues: if my understanding were correct, return 1 from
>> >> madvise_free_huge_pmd means the following processing for the PMD should
>> >> be skipped, while return 0 means the following processing is still
>> >> needed.  So the function should return 0 only if the THP is split
>> >> successfully or the PMD is not trans huge.  But the pmd_trans_unstable
>> >> after madvise_free_huge_pmd guarantee the following processing will be
>> >> skipped for huge PMD.  So current code can run properly.  But if my
>> >> understanding were correct, we can clean up return code of
>> >> madvise_free_huge_pmd accordingly.
>> >
>> > I like your clean up. Just a minor comment below.
>> >
>> >> 
>> >> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
>> >> ---
>> >>  mm/huge_memory.c | 7 +------
>> >>  1 file changed, 1 insertion(+), 6 deletions(-)
>> >> 
>> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> >> index 2ad52d5..64dc95d 100644
>> >> --- a/mm/huge_memory.c
>> >> +++ b/mm/huge_memory.c
>> >
>> > First of all, let's change ret from int to bool.
>> > And then, add description in the function entry.
>> 
>> Yes.  bool looks better than int.
>> 
>> > /*
>> >  * Return true if we do MADV_FREE successfully on entire pmd page.
>> >  * Otherwise, return false.
>> >  */
>> 
>> This way, we need to return false if we failed to split huge page, this
>> will cause unnecessary pmd_trans_unstable check.  How about to change
>> the comments to
>
> I focused the function name "madvise_free_huge_pmd". IOW, the function
> should free huge pmd page. If it is successful, done. Otherwise, next
> routines should handle it.
>
> If it fail to split, pmd_trans_unstable will check it and return.
> I don't think it's heavy operation to affect performance so rather
> than making function fast, I wanted to make it simple by following
> function name.

Reasonable.  Will do that.

Best Regards,
Huang, Ying

>> 
>> /*
>>  * Return true if we finished processing entire pmd page and needn't
>>  * fall back pte processing.  Otherwise, return false.
>>  */
>> 
>> Best Regards,
>> Huang, Ying
>> 
>> > And do not set to 1 if it is huge_zero_pmd but just goto out to
>> > return false.
>> >
>> > Thanks!
>> >
>> >> @@ -1655,14 +1655,9 @@ int madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>> >>  	if (next - addr != HPAGE_PMD_SIZE) {
>> >>  		get_page(page);
>> >>  		spin_unlock(ptl);
>> >> -		if (split_huge_page(page)) {
>> >> -			put_page(page);
>> >> -			unlock_page(page);
>> >> -			goto out_unlocked;
>> >> -		}
>> >> +		split_huge_page(page);
>> >>  		put_page(page);
>> >>  		unlock_page(page);
>> >> -		ret = 1;
>> >>  		goto out_unlocked;
>> >>  	}
>> >>  
>> >> -- 
>> >> 2.8.1
>> >> 
>> >> --
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