Minchan Kim <minchan@xxxxxxxxxx> writes:
> On Fri, Jun 17, 2016 at 12:45:43PM -0700, Huang, Ying wrote:
>> Minchan Kim <minchan@xxxxxxxxxx> writes:
>>
>> > Hi,
>> >
>> > On Thu, Jun 16, 2016 at 08:03:54PM -0700, Huang, Ying wrote:
>> >> From: Huang Ying <ying.huang@xxxxxxxxx>
>> >>
>> >> 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@xxxxxxxxx>
>> >> ---
>> >> 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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