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    Subject[PATCHv6 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path
    Date
    FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
    going to be given to hardware and can't move. It would truncate CMA
    permanently and should be excluded.

    In gup slow path, slow path, where
    __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
    but in fast path, there lacks such a check, which means a possible leak of
    CMA page to longterm pinned.

    Place a check in try_grab_compound_head() in the fast path to fix the leak,
    and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
    migrate the page.

    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: Christoph Hellwig <hch@infradead.org>
    Cc: Shuah Khan <shuah@kernel.org>
    Cc: Jason Gunthorpe <jgg@ziepe.ca>
    To: linux-mm@kvack.org
    Cc: linux-kernel@vger.kernel.org
    ---
    mm/gup.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    diff --git a/mm/gup.c b/mm/gup.c
    index 9df77b1..78132cf 100644
    --- a/mm/gup.c
    +++ b/mm/gup.c
    @@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
    int orig_refs = refs;

    /*
    + * 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 subpage.
    + */
    + if (unlikely(flags & FOLL_LONGTERM) &&
    + is_migrate_cma_page(page))
    + return NULL;
    +
    + /*
    * When pinning a compound page of order > 1 (which is what
    * hpage_pincount_available() checks for), use an exact count to
    * track it, via hpage_pincount_add/_sub().
    --
    2.7.5
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