RCU Archive on lore.kernel.org
 help / color / Atom feed
From: Uladzislau Rezki <urezki@gmail.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	RCU <rcu@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 4/7] rcu/tree: support reclaim for head-less object
Date: Mon, 30 Mar 2020 14:48:41 +0200
Message-ID: <20200330124841.GA15431@pc636> (raw)
In-Reply-To: <20200329225610.GA102186@google.com>

> 
> Very nice work, Vlad! And beautifully split! Makes it easy to review! One
> comment below but otherwise patches 1-4 look good to me, I will look at
> others as well now. I have some patches on top of the series, mostly little
> clean ups which I will send together with yours.
> 
Thanks, Joel!


> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  kernel/rcu/tree.c | 94 +++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 86 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 869a72e25d38..5a64c92feafc 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2892,19 +2892,34 @@ static void kfree_rcu_work(struct work_struct *work)
> >  	 * when we could not allocate a bulk array.
> >  	 *
> >  	 * Under that condition an object is queued to the
> > -	 * list instead.
> > +	 * list instead. Please note that head-less objects
> > +	 * have dynamically attached rcu_head, so they also
> > +	 * contain a back-pointer that has to be freed.
> >  	 */
> >  	for (; head; head = next) {
> >  		unsigned long offset = (unsigned long)head->func;
> > -		void *ptr = (void *)head - offset;
> > +		bool headless;
> > +		void *ptr;
> >  
> >  		next = head->next;
> > +
> > +		/* We tag the headless object, if so adjust offset. */
> > +		headless = (((unsigned long) head - offset) & BIT(0));
> > +		if (headless)
> > +			offset -= 1;
> 
> Just to be sure, can vmalloc() ever allocate an object at an odd valued
> memory address? I'm not fully sure looking at vmalloc code whether this is
> the case.
>
No. It must be PAGE_ALIGNED(addr). 

> 
> As per the tagging, allocated objects have to at least at a 2-byte boundary
> for the pointer's BIT(0) to be available. If that's not the case, we need to
> add a warning to the code at a bare minimum.
> 
> Another approach which is better I think is to add the tag to the offset
> itself. So if the offset is > LONG_MAX / 2 or something like that, then
> assume it is headless, and override offset to sizeof(unsigned long *) in that
> case. Then you would arrive at the correct pointer for the wrapper. That
> would take care of the case where in the future, either SLAB or vmalloc()
> ends up returning pointers that are only byte-aligned.
> 
> Thoughts?
> 
I saw your https://lkml.org/lkml/2020/3/29/480. I think it is OK,
and we can go your way. There is advantage that it is tolerate to
any alignment :)

Thanks!

--
Vlad Rezki

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-23 11:36 [PATCH 0/7] Headless support in the kvfree_rcu() Uladzislau Rezki (Sony)
2020-03-23 11:36 ` [PATCH 1/7] rcu/tree: simplify KFREE_BULK_MAX_ENTR macro Uladzislau Rezki (Sony)
2020-03-23 11:36 ` [PATCH 2/7] rcu/tree: maintain separate array for vmalloc ptrs Uladzislau Rezki (Sony)
2020-03-23 11:36 ` [PATCH 3/7] rcu/tree: introduce expedited_drain flag Uladzislau Rezki (Sony)
2020-03-23 11:36 ` [PATCH 4/7] rcu/tree: support reclaim for head-less object Uladzislau Rezki (Sony)
2020-03-29 22:56   ` Joel Fernandes
2020-03-30 12:48     ` Uladzislau Rezki [this message]
2020-03-23 11:36 ` [PATCH 5/7] rcu/tiny: move kvfree_call_rcu() out of header Uladzislau Rezki (Sony)
2020-03-23 11:36 ` [PATCH 6/7] rcu/tiny: support reclaim for head-less object Uladzislau Rezki (Sony)
2020-03-30  0:56   ` Joel Fernandes
2020-03-30 14:42     ` Uladzislau Rezki
2020-03-23 11:36 ` [PATCH 7/7] rcu: support headless variant in the kvfree_rcu() Uladzislau Rezki (Sony)

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=20200330124841.GA15431@pc636 \
    --to=urezki@gmail.com \
    --cc=akpm@linux-foundation.org \
    --cc=joel@joelfernandes.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=oleksiy.avramchenko@sonymobile.com \
    --cc=paulmck@kernel.org \
    --cc=rcu@vger.kernel.org \
    --cc=rostedt@goodmis.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

RCU Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/rcu/0 rcu/git/0.git

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

Example config snippet for mirrors

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


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