Re: [PATCH -mm] mm, swap: Fix THP swap out

[Ming Lei]

Hi Huang Ying,

On Mon, Jun 24, 2019 at 10:23:36AM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> 0-Day test system reported some OOM regressions for several
> THP (Transparent Huge Page) swap test cases.  These regressions are
> bisected to 6861428921b5 ("block: always define BIO_MAX_PAGES as
> 256").  In the commit, BIO_MAX_PAGES is set to 256 even when THP swap
> is enabled.  So the bio_alloc(gfp_flags, 512) in get_swap_bio() may
> fail when swapping out THP.  That causes the OOM.
> 
> As in the patch description of 6861428921b5 ("block: always define
> BIO_MAX_PAGES as 256"), THP swap should use multi-page bvec to write
> THP to swap space.  So the issue is fixed via doing that in
> get_swap_bio().
> 
> BTW: I remember I have checked the THP swap code when
> 6861428921b5 ("block: always define BIO_MAX_PAGES as 256") was merged,
> and thought the THP swap code needn't to be changed.  But apparently,
> I was wrong.  I should have done this at that time.
> 
> Fixes: 6861428921b5 ("block: always define BIO_MAX_PAGES as 256")
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> ---
>  mm/page_io.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 2e8019d0e048..4ab997f84061 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -29,10 +29,9 @@
>  static struct bio *get_swap_bio(gfp_t gfp_flags,
>  				struct page *page, bio_end_io_t end_io)
>  {
> -	int i, nr = hpage_nr_pages(page);
>  	struct bio *bio;
>  
> -	bio = bio_alloc(gfp_flags, nr);
> +	bio = bio_alloc(gfp_flags, 1);
>  	if (bio) {
>  		struct block_device *bdev;
>  
> @@ -41,9 +40,7 @@ static struct bio *get_swap_bio(gfp_t gfp_flags,
>  		bio->bi_iter.bi_sector <<= PAGE_SHIFT - 9;
>  		bio->bi_end_io = end_io;
>  
> -		for (i = 0; i < nr; i++)
> -			bio_add_page(bio, page + i, PAGE_SIZE, 0);

bio_add_page() supposes to work, just wondering why it doesn't recently.

Could you share me one test case for reproducing it?

> -		VM_BUG_ON(bio->bi_iter.bi_size != PAGE_SIZE * nr);
> +		__bio_add_page(bio, page, PAGE_SIZE * hpage_nr_pages(page), 0);
>  	}
>  	return bio;

Actually the above code can be simplified as:

diff --git a/mm/page_io.c b/mm/page_io.c
index 2e8019d0e048..c20b4189d0a1 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -29,7 +29,7 @@
 static struct bio *get_swap_bio(gfp_t gfp_flags,
 				struct page *page, bio_end_io_t end_io)
 {
-	int i, nr = hpage_nr_pages(page);
+	int nr = hpage_nr_pages(page);
 	struct bio *bio;
 
 	bio = bio_alloc(gfp_flags, nr);
@@ -41,8 +41,7 @@ static struct bio *get_swap_bio(gfp_t gfp_flags,
 		bio->bi_iter.bi_sector <<= PAGE_SHIFT - 9;
 		bio->bi_end_io = end_io;
 
-		for (i = 0; i < nr; i++)
-			bio_add_page(bio, page + i, PAGE_SIZE, 0);
+		bio_add_page(bio, page, PAGE_SIZE * nr, 0);
 		VM_BUG_ON(bio->bi_iter.bi_size != PAGE_SIZE * nr);
 	}
 	return bio;


Thanks,
Ming