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

[Huang\, Ying]

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.

Best Regards,
Huang, Ying

> 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