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    SubjectRe: [PATCH v9 23/25] mm/gup: track FOLL_PIN pages
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    On 12/11/19 3:28 AM, Jan Kara wrote:
    ...
    >
    > The patch looks mostly good to me now. Just a few smaller comments below.
    >
    >> Suggested-by: Jan Kara <jack@suse.cz>
    >> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
    >> Reviewed-by: Jan Kara <jack@suse.cz>
    >> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
    >> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
    >
    > I think you inherited here the Reviewed-by tags from the "add flags" patch
    > you've merged into this one but that's not really fair since this patch
    > does much more... In particular I didn't give my Reviewed-by tag for this
    > patch yet.

    OK, I've removed those reviewed-by's. (I felt bad about dropping them, after
    people had devoted time to reviewing, but I do see that it's wrong to imply
    that they've reviewed this much much larger thing.)

    ...
    >
    > I somewhat wonder about the asymmetry of try_grab_compound_head() vs
    > try_grab_page() in the treatment of 'flags'. How costly would it be to make
    > them symmetric (i.e., either set FOLL_GET for try_grab_compound_head()
    > callers or make sure one of FOLL_GET, FOLL_PIN is set for try_grab_page())?
    >
    > Because this difference looks like a subtle catch in the long run...

    Done. It is only a modest code-level change, at least the way I've done it, which is
    setting FOLL_GET for try_grab_compound_head(). In order to do that, I set
    it at the top of the internal gup fast calling stacks, which is actually a good
    design anyway: gup fast is logically doing FOLL_GET in all cases. So setting
    the flag internally is accurate and consistent with the overall design.


    > ...
    >
    >> @@ -1522,8 +1536,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
    >> skip_mlock:
    >> page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
    >> VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
    >> - if (flags & FOLL_GET)
    >> - get_page(page);
    >> + if (!try_grab_page(page, flags))
    >> + page = ERR_PTR(-EFAULT);
    >
    > I think you need to also move the try_grab_page() earlier in the function.
    > At this point the page may be marked as mlocked and you'd need to undo that
    > in case try_grab_page() fails.


    OK, I've moved it up, adding a "subpage" variable in order to make that work.

    >
    >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
    >> index ac65bb5e38ac..0aab6fe0072f 100644
    >> --- a/mm/hugetlb.c
    >> +++ b/mm/hugetlb.c
    >> @@ -4356,7 +4356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
    >> same_page:
    >> if (pages) {
    >> pages[i] = mem_map_offset(page, pfn_offset);
    >> - get_page(pages[i]);
    >> + if (!try_grab_page(pages[i], flags)) {
    >> + spin_unlock(ptl);
    >> + remainder = 0;
    >> + err = -ENOMEM;
    >> + WARN_ON_ONCE(1);
    >> + break;
    >> + }
    >> }
    >
    > This function does a refcount overflow check early so that it doesn't have
    > to do try_get_page() here. So that check can be now removed when you do
    > try_grab_page() here anyway since that early check seems to be just a tiny
    > optimization AFAICT.
    >
    > Honza
    >

    Yes. I've removed it, good spot.


    thanks,
    --
    John Hubbard
    NVIDIA

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