Re: [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge_pmd return value after splitting

Minchan Kim [off-list ref] writes:

On Fri, Jun 17, 2016 at 12:45:43PM -0700, Huang, Ying wrote:

quoted

Minchan Kim [off-list ref] writes:

quoted

Hi,

On Thu, Jun 16, 2016 at 08:03:54PM -0700, Huang, Ying wrote:

quoted

From: Huang Ying <redacted>

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!

quoted

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.

quoted

Signed-off-by: "Huang, Ying" <redacted>
---
 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.

quoted

/*
 * 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

quoted

/*
 * Return true if we finished processing entire pmd page and needn't
 * fall back pte processing.  Otherwise, return false.
 */

Best Regards,
Huang, Ying

quoted

And do not set to 1 if it is huge_zero_pmd but just goto out to
return false.

Thanks!

quoted

@@ -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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>