Re: [PATCHv5 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

Ira Weiny
2020-02-28T22:34:50Z

On Fri, Feb 28, 2020 at 07:32:29PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM suggests a pin which is going to be given to hardware and
> can't move. It would truncate CMA permanently and should be excluded.

I don't understand what is 'truncated' here?

I generally agree with Jason that this is going to be confusing to the user.

Ira

>
> FOLL_LONGTERM has already been checked in the slow path, but not checked in
> the fast path, which means a possible leak of CMA page to longterm pinned
> requirement through this crack.
>
> Place a check in try_get_compound_head() in the fast path.
>
> Some note about the check:
> Huge page's subpages have the same migrate type due to either
> allocation from a free_list[] or alloc_contig_range() with param
> MIGRATE_MOVABLE. So it is enough to check on a single subpage
> by is_migrate_cma_page(subpage)
>
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Shuah Khan <shuah@kernel.org>
> To: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> mm/gup.c | 26 +++++++++++++++++++-------
> 1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index cd8075e..f0d6804 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -33,9 +33,21 @@ struct follow_page_context {
> * Return the compound head page with ref appropriately incremented,
> * or NULL if that failed.
> */
> -static inline struct page *try_get_compound_head(struct page *page, int refs)
> +static inline struct page *try_get_compound_head(struct page *page, int refs,
> + unsigned int flags)
> {
> - struct page *head = compound_head(page);
> + struct page *head;
> +
> + /*
> + * Huge page's subpages have the same migrate type due to either
> + * allocation from a free_list[] or alloc_contig_range() with param
> + * MIGRATE_MOVABLE. So it is enough to check on a single subpage.
> + */
> + if (unlikely(flags & FOLL_LONGTERM) &&
> + is_migrate_cma_page(page))
> + return NULL;
> +
> + head = compound_head(page);
>
> if (WARN_ON_ONCE(page_ref_count(head) < 0))
> return NULL;
> @@ -1908,7 +1920,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
> page = pte_page(pte);
>
> - head = try_get_compound_head(page, 1);
> + head = try_get_compound_head(page, 1, flags);
> if (!head)
> goto pte_unmap;
>
> @@ -2083,7 +2095,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
> page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
> refs = record_subpages(page, addr, end, pages + *nr);
>
> - head = try_get_compound_head(head, refs);
> + head = try_get_compound_head(head, refs, flags);
> if (!head)
> return 0;
>
> @@ -2142,7 +2154,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> refs = record_subpages(page, addr, end, pages + *nr);
>
> - head = try_get_compound_head(pmd_page(orig), refs);
> + head = try_get_compound_head(pmd_page(orig), refs, flags);
> if (!head)
> return 0;
>
> @@ -2174,7 +2186,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> refs = record_subpages(page, addr, end, pages + *nr);
>
> - head = try_get_compound_head(pud_page(orig), refs);
> + head = try_get_compound_head(pud_page(orig), refs, flags);
> if (!head)
> return 0;
>
> @@ -2203,7 +2215,7 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
> refs = record_subpages(page, addr, end, pages + *nr);
>
> - head = try_get_compound_head(pgd_page(orig), refs);
> + head = try_get_compound_head(pgd_page(orig), refs, flags);
> if (!head)
> return 0;
>
> --
> 2.7.5
>
>