LKML Archive on lore.kernel.org
 help / color / Atom feed
From: John Hubbard <jhubbard@nvidia.com>
To: Jason Gunthorpe <jgg@nvidia.com>, <linux-kernel@vger.kernel.org>,
	Peter Xu <peterx@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>, Hugh Dickins <hughd@google.com>,
	Jan Kara <jack@suse.cz>, Jann Horn <jannh@google.com>,
	Kirill Shutemov <kirill@shutemov.name>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2 1/2] mm: reorganize internal_get_user_pages_fast()
Date: Fri, 30 Oct 2020 14:31:54 -0700
Message-ID: <aba86298-cced-bdb7-542f-678646468502@nvidia.com> (raw)
In-Reply-To: <1-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>

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@nvidia.com>
> ---
>   mm/gup.c | 99 ++++++++++++++++++++++++++++++--------------------------
>   1 file changed, 54 insertions(+), 45 deletions(-)

Everything still looks correct.

Reviewed-by: John Hubbard <jhubbard@nvidia.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(&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;
> -	}
> +	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
> 


  parent reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 14:46 ` Jason Gunthorpe
2020-10-30 16:29   ` Jan Kara
2020-10-30 21:31   ` John Hubbard [this message]
2020-10-30 22:36   ` Peter Xu
2020-11-02 22:19 ` [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range() Ahmed S. Darwish
2020-11-02 22:39   ` Linus Torvalds
2020-11-02 23:18     ` Ahmed S. Darwish
     [not found] ` <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 16:51   ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Jan Kara
     [not found]     ` <20201030170226.GF2620339@nvidia.com>
2020-11-02  8:31       ` Jan Kara
2020-10-30 22:52   ` Peter Xu
     [not found]     ` <20201030235121.GQ2620339@nvidia.com>
2020-10-31 15:26       ` Peter Xu
2020-11-03  0:33         ` Ahmed S. Darwish
2020-11-03  0:17       ` Ahmed S. Darwish
     [not found]         ` <20201103002532.GL2620339@nvidia.com>
2020-11-03  0:41           ` Ahmed S. Darwish
2020-11-03  2:20             ` John Hubbard
2020-11-03  6:52               ` Ahmed S. Darwish
2020-11-03 17:40                 ` Linus Torvalds
2020-11-04  1:32                   ` Ahmed S. Darwish
2020-11-04  2:01                     ` John Hubbard
2020-11-04  3:17                       ` Ahmed S. Darwish
2020-11-04 18:38                         ` Linus Torvalds
2020-11-04 19:54                           ` Ahmed S. Darwish
2020-12-09 18:38                           ` [tip: locking/core] seqlock: Prefix internal seqcount_t-only macros with a "do_" tip-bot2 for Ahmed S. Darwish
2020-12-09 18:38                           ` [tip: locking/core] seqlock: kernel-doc: Specify when preemption is automatically altered tip-bot2 for Ahmed S. Darwish
2020-11-10 11:53                         ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Peter Zijlstra
2020-12-03 10:35                           ` [tip: locking/core] seqlock: Rename __seqprop() users tip-bot2 for Peter Zijlstra
2020-11-03 17:03         ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Peter Xu
2020-11-02 23:58   ` Ahmed S. Darwish

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=aba86298-cced-bdb7-542f-678646468502@nvidia.com \
    --to=jhubbard@nvidia.com \
    --cc=aarcange@redhat.com \
    --cc=akpm@linux-foundation.org \
    --cc=hch@lst.de \
    --cc=hughd@google.com \
    --cc=jack@suse.cz \
    --cc=jannh@google.com \
    --cc=jgg@nvidia.com \
    --cc=kirill@shutemov.name \
    --cc=ktkhai@virtuozzo.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=oleg@redhat.com \
    --cc=peterx@redhat.com \
    --cc=torvalds@linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lkml.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lkml.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lkml.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lkml.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lkml.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lkml.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lkml.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lkml.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lkml.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lkml.kernel.org/lkml/9 lkml/git/9.git
	git clone --mirror https://lkml.kernel.org/lkml/10 lkml/git/10.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lkml.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git