Messages in this thread | | | Subject | Re: [PATCH v9 23/25] mm/gup: track FOLL_PIN pages | From | John Hubbard <> | Date | Wed, 11 Dec 2019 21:53:45 -0800 |
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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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