Hi Steve,
> Subject: [PATCH V1 2/5] mm/hugetlb: fix memfd_pin_folios free_huge_pages
> leak
>
> memfd_pin_folios followed by unpin_folios fails to restore
> free_huge_pages if the pages were not already faulted in, because the
> folio refcount for pages created by memfd_alloc_folio never goes to 0.
> memfd_pin_folios needs another folio_put to undo the folio_try_get
> below:
>
> memfd_alloc_folio()
> alloc_hugetlb_folio_nodemask()
> dequeue_hugetlb_folio_nodemask()
> dequeue_hugetlb_folio_node_exact()
> folio_ref_unfreeze(folio, 1); ; adds 1 refcount
> folio_try_get() ; adds 1 refcount
I wonder if it is more optimal to skip the folio_try_get() above.
I think it (folio_try_get) was probably added because I didn't realize that
alloc_hugetlb_folio_nodemask() already adds a reference.
> hugetlb_add_to_page_cache() ; adds 512 refcount (on x86)
>
> With the fix, after memfd_pin_folios + unpin_folios, the refcount for
> the (unfaulted) page is 512, which is correct, as the refcount for a
> faulted unpinned page is 513.
>
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning
> memfd folios")
>
> Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> ---
> mm/gup.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 54d0dc3..5b92f1d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3618,7 +3618,7 @@ long memfd_pin_folios(struct file *memfd, loff_t
> start, loff_t end,
> pgoff_t start_idx, end_idx, next_idx;
> struct folio *folio = NULL;
> struct folio_batch fbatch;
> - struct hstate *h;
> + struct hstate *h = NULL;
> long ret = -EINVAL;
>
> if (start < 0 || start > end || !max_folios)
> @@ -3662,6 +3662,8 @@ long memfd_pin_folios(struct file *memfd, loff_t
> start, loff_t end,
> &fbatch);
> if (folio) {
> folio_put(folio);
> + if (h)
> + folio_put(folio);
If we stick with this change, I guess there needs to be an additional
folio_put() in memfd_alloc_folio() as well if adding the folio to the
page cache fails.
Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Thanks,
Vivek
> folio = NULL;
> }
>
> --
> 1.8.3.1
