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    Subject[PATCH 5.10 446/717] mm/gup: reorganize internal_get_user_pages_fast()
    Date
    From: Jason Gunthorpe <jgg@nvidia.com>

    [ Upstream commit c28b1fc70390df32e29991eedd52bd86e7aba080 ]

    Patch series "Add a seqcount between gup_fast and copy_page_range()", v4.

    As discussed and suggested by Linus use a seqcount to close the small race
    between gup_fast and copy_page_range().

    Ahmed confirms that raw_write_seqcount_begin() is the correct API to use
    in this case and it doesn't trigger any lockdeps.

    I was able to test it using two threads, one forking and the other using
    ibv_reg_mr() to trigger GUP fast. Modifying copy_page_range() to sleep
    made the window large enough to reliably hit to test the logic.

    This patch (of 2):

    The next patch in this series makes the lockless flow a little more
    complex, so move the entire block into a new function and remove a level
    of indention. Tidy a bit of cruft:

    - addr is always the same as start, so use start

    - Use the modern check_add_overflow() for computing end = start + len

    - nr_pinned/pages << PAGE_SHIFT needs the LHS to be unsigned long to
    avoid shift overflow, make the variables unsigned long to avoid coding
    casts in both places. nr_pinned was missing its cast

    - The handling of ret and nr_pinned can be streamlined a bit

    No functional change.

    Link: https://lkml.kernel.org/r/0-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
    Link: https://lkml.kernel.org/r/1-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Reviewed-by: John Hubbard <jhubbard@nvidia.com>
    Reviewed-by: Peter Xu <peterx@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/gup.c | 99 ++++++++++++++++++++++++++++++--------------------------
    1 file changed, 54 insertions(+), 45 deletions(-)

    diff --git a/mm/gup.c b/mm/gup.c
    index 98eb8e6d2609c..c7e24301860ab 100644
    --- a/mm/gup.c
    +++ b/mm/gup.c
    @@ -2677,13 +2677,43 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
    return ret;
    }

    -static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
    +static unsigned long lockless_pages_from_mm(unsigned long start,
    + unsigned long end,
    + unsigned int gup_flags,
    + struct page **pages)
    +{
    + unsigned long flags;
    + int nr_pinned = 0;
    +
    + if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
    + !gup_fast_permitted(start, end))
    + return 0;
    +
    + /*
    + * Disable interrupts. The nested form is used, in order to allow full,
    + * general purpose use of this routine.
    + *
    + * With interrupts disabled, we block page table pages from being freed
    + * from under us. See struct mmu_table_batch comments in
    + * include/asm-generic/tlb.h for more details.
    + *
    + * We do not adopt an rcu_read_lock() here as we also want to block IPIs
    + * that come from THPs splitting.
    + */
    + local_irq_save(flags);
    + gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
    + local_irq_restore(flags);
    + return nr_pinned;
    +}
    +
    +static int internal_get_user_pages_fast(unsigned long start,
    + unsigned long nr_pages,
    unsigned int gup_flags,
    struct page **pages)
    {
    - unsigned long addr, len, end;
    - unsigned long flags;
    - int nr_pinned = 0, ret = 0;
    + unsigned long len, end;
    + unsigned long nr_pinned;
    + int ret;

    if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
    FOLL_FORCE | FOLL_PIN | FOLL_GET |
    @@ -2697,54 +2727,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
    might_lock_read(&current->mm->mmap_lock);

    start = untagged_addr(start) & PAGE_MASK;
    - addr = start;
    - len = (unsigned long) nr_pages << PAGE_SHIFT;
    - end = start + len;
    -
    - if (end <= start)
    + len = nr_pages << PAGE_SHIFT;
    + if (check_add_overflow(start, len, &end))
    return 0;
    if (unlikely(!access_ok((void __user *)start, len)))
    return -EFAULT;

    - /*
    - * Disable interrupts. The nested form is used, in order to allow
    - * full, general purpose use of this routine.
    - *
    - * With interrupts disabled, we block page table pages from being
    - * freed from under us. See struct mmu_table_batch comments in
    - * include/asm-generic/tlb.h for more details.
    - *
    - * We do not adopt an rcu_read_lock(.) here as we also want to
    - * block IPIs that come from THPs splitting.
    - */
    - if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
    - unsigned long fast_flags = gup_flags;
    -
    - local_irq_save(flags);
    - gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);
    - local_irq_restore(flags);
    - ret = nr_pinned;
    - }
    -
    - if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) {
    - /* Try to get the remaining pages with get_user_pages */
    - start += nr_pinned << PAGE_SHIFT;
    - pages += nr_pinned;
    + nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
    + if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
    + return nr_pinned;

    - ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned,
    - gup_flags, pages);
    -
    - /* Have to be a bit careful with return values */
    - if (nr_pinned > 0) {
    - if (ret < 0)
    - ret = nr_pinned;
    - else
    - ret += nr_pinned;
    - }
    + /* Slow path: try to get the remaining pages with get_user_pages */
    + start += nr_pinned << PAGE_SHIFT;
    + pages += nr_pinned;
    + ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
    + pages);
    + if (ret < 0) {
    + /*
    + * The caller has to unpin the pages we already pinned so
    + * returning -errno is not an option
    + */
    + if (nr_pinned)
    + return nr_pinned;
    + return ret;
    }
    -
    - return ret;
    + return ret + nr_pinned;
    }
    +
    /**
    * get_user_pages_fast_only() - pin user pages in memory
    * @start: starting user address
    --
    2.27.0


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