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

[Ming Lei]

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.


thanks,
Ming