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SubjectRe: [PATCH -mm] mm, swap: Fix THP swap out
Date
Ming Lei <ming.lei@redhat.com> writes:

> Hi Huang Ying,
>
> On Mon, Jun 24, 2019 at 10:23:36AM +0800, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>>
>> 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@intel.com>
>> Cc: Ming Lei <ming.lei@redhat.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
>> ---
>> 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

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