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    SubjectRe: [PATCH 1/2] mm: gup: Re-pin pages in case of trying several times to migrate
    Date
    Baolin Wang <baolin.wang@linux.alibaba.com> writes:

    > The migrate_pages() will return the number of {normal page, THP, hugetlb}
    > that were not migrated, or an error code. That means it can still return
    > the number of failure count, though the pages have been migrated
    > successfully with several times re-try.

    If my understanding were correct, if pages are migrated successfully
    after several times re-tries, the return value will be 0. There's one
    possibility for migrate_pages() to return non-zero but all pages are
    migrated. That is, when THP is split and all subpages are migrated
    successfully.

    > So we should not use the return value of migrate_pages() to determin
    > if there are pages are failed to migrate. Instead we can validate the
    > 'movable_page_list' to see if there are pages remained in the list,
    > which are failed to migrate. That can mitigate the failure of longterm
    > pinning.

    Another choice is to use a special return value for split THP + success
    migration. But I'm fine to use list_empty(return_pages).

    > Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
    > ---
    > mm/gup.c | 7 ++++---
    > 1 file changed, 4 insertions(+), 3 deletions(-)
    >
    > diff --git a/mm/gup.c b/mm/gup.c
    > index 5182aba..bd8cfcd 100644
    > --- a/mm/gup.c
    > +++ b/mm/gup.c
    > @@ -1914,9 +1914,10 @@ static int migrate_longterm_unpinnable_pages(
    > .gfp_mask = GFP_USER | __GFP_NOWARN,
    > };
    >
    > - if (migrate_pages(movable_page_list, alloc_migration_target,
    > - NULL, (unsigned long)&mtc, MIGRATE_SYNC,
    > - MR_LONGTERM_PIN, NULL)) {
    > + ret = migrate_pages(movable_page_list, alloc_migration_target,
    > + NULL, (unsigned long)&mtc, MIGRATE_SYNC,
    > + MR_LONGTERM_PIN, NULL);
    > + if (ret < 0 || !list_empty(movable_page_list)) {

    It seems that !list_empty() is sufficient here.

    > ret = -ENOMEM;

    Why change the error code? I don't think it's a good idea to do that.

    > goto err;
    > }

    Best Regards,
    Huang, Ying

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