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

[Huang\, Ying]

Ming Lei <ming.lei@xxxxxxxxxx> writes:

> On Mon, Jun 24, 2019 at 12:44:41PM +0800, Huang, Ying wrote:
>> Ming Lei <ming.lei@xxxxxxxxxx> writes:
>> 
>> > 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.
>> 
>> Yes.  Just checked and bio_add_page() works too.  I should have used
>> that.  The problem isn't bio_add_page(), but bio_alloc(), because nr ==
>> 512 > 256, mempool cannot be used during swapout, so swapout will fail.
>
> Then we can pass 1 to bio_alloc(), together with single bio_add_page()
> for making the code more readable.
>

Yes.  Will send out v2 to replace __bio_add_page() with bio_add_page().

Best Regards,
Huang, Ying