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Message-ID: <aba86298-cced-bdb7-542f-678646468502@nvidia.com>
Date: Fri, 30 Oct 2020 14:31:54 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>, <linux-kernel@...r.kernel.org>,
Peter Xu <peterx@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
CC: Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@....de>,
Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
Jann Horn <jannh@...gle.com>,
Kirill Shutemov <kirill@...temov.name>,
Kirill Tkhai <ktkhai@...tuozzo.com>,
Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/2] mm: reorganize internal_get_user_pages_fast()
On 10/30/20 7:46 AM, Jason Gunthorpe wrote:
> 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.
>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> ---
> mm/gup.c | 99 ++++++++++++++++++++++++++++++--------------------------
> 1 file changed, 54 insertions(+), 45 deletions(-)
Everything still looks correct.
Reviewed-by: John Hubbard <jhubbard@...dia.com>
thanks,
--
John Hubbard
NVIDIA
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 102877ed77a4b4..150cc962c99201 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2671,13 +2671,43 @@ 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 |
> @@ -2691,54 +2721,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
> might_lock_read(¤t->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;
> - }
> + nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
> + if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
> + return 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;
> -
> - 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
>
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