Re: [PATCH v9 23/25] mm/gup: track FOLL_PIN pages

From: John Hubbard
Date: Thu Dec 12 2019 - 00:56:41 EST

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@xxxxxxx>
Suggested-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>

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,
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,
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;
+ 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.


Yes. I've removed it, good spot.

John Hubbard