linux-kernel - Re: [PATCH] device-dax: Fix range release

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Date:   Sat, 19 Dec 2020 10:51:34 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Jane Chu <jane.chu@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] device-dax: Fix range release

On Fri, Dec 18, 2020 at 11:46 PM Leizhen (ThunderTown)
<thunder.leizhen@...wei.com> wrote:
>
>
>
> On 2020/12/19 10:41, Dan Williams wrote:
> > There are multiple locations that open-code the release of the last
> > range in a device-dax instance. Consolidate this into a new
> > dev_dax_trim_range() helper.
> >
> > This also addresses a kmemleak report:
> >
> > # cat /sys/kernel/debug/kmemleak
> > [..]
> > unreferenced object 0xffff976bd46f6240 (size 64):
> >    comm "ndctl", pid 23556, jiffies 4299514316 (age 5406.733s)
> >    hex dump (first 32 bytes):
> >      00 00 00 00 00 00 00 00 00 00 20 c3 37 00 00 00  .......... .7...
> >      ff ff ff 7f 38 00 00 00 00 00 00 00 00 00 00 00  ....8...........
> >    backtrace:
> >      [<00000000064003cf>] __kmalloc_track_caller+0x136/0x379
> >      [<00000000d85e3c52>] krealloc+0x67/0x92
> >      [<00000000d7d3ba8a>] __alloc_dev_dax_range+0x73/0x25c
> >      [<0000000027d58626>] devm_create_dev_dax+0x27d/0x416
> >      [<00000000434abd43>] __dax_pmem_probe+0x1c9/0x1000 [dax_pmem_core]
> >      [<0000000083726c1c>] dax_pmem_probe+0x10/0x1f [dax_pmem]
> >      [<00000000b5f2319c>] nvdimm_bus_probe+0x9d/0x340 [libnvdimm]
> >      [<00000000c055e544>] really_probe+0x230/0x48d
> >      [<000000006cabd38e>] driver_probe_device+0x122/0x13b
> >      [<0000000029c7b95a>] device_driver_attach+0x5b/0x60
> >      [<0000000053e5659b>] bind_store+0xb7/0xc3
> >      [<00000000d3bdaadc>] drv_attr_store+0x27/0x31
> >      [<00000000949069c5>] sysfs_kf_write+0x4a/0x57
> >      [<000000004a8b5adf>] kernfs_fop_write+0x150/0x1e5
> >      [<00000000bded60f0>] __vfs_write+0x1b/0x34
> >      [<00000000b92900f0>] vfs_write+0xd8/0x1d1
> >
> > Reported-by: Jane Chu <jane.chu@...cle.com>
> > Cc: Zhen Lei <thunder.leizhen@...wei.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > ---
> >  drivers/dax/bus.c |   44 +++++++++++++++++++++-----------------------
> >  1 file changed, 21 insertions(+), 23 deletions(-)
> >
> > diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> > index 9761cb40d4bb..720cd140209f 100644
> > --- a/drivers/dax/bus.c
> > +++ b/drivers/dax/bus.c
> > @@ -367,19 +367,28 @@ void kill_dev_dax(struct dev_dax *dev_dax)
> >  }
> >  EXPORT_SYMBOL_GPL(kill_dev_dax);
> >
> > -static void free_dev_dax_ranges(struct dev_dax *dev_dax)
> > +static void trim_dev_dax_range(struct dev_dax *dev_dax)
> >  {
> > +     int i = dev_dax->nr_range - 1;
> > +     struct range *range = &dev_dax->ranges[i].range;
> >       struct dax_region *dax_region = dev_dax->region;
> > -     int i;
> >
> >       device_lock_assert(dax_region->dev);
> > -     for (i = 0; i < dev_dax->nr_range; i++) {
> > -             struct range *range = &dev_dax->ranges[i].range;
> > -
> > -             __release_region(&dax_region->res, range->start,
> > -                             range_len(range));
> > +     dev_dbg(&dev_dax->dev, "delete range[%d]: %#llx:%#llx\n", i,
> > +             (unsigned long long)range->start,
> > +             (unsigned long long)range->end);
> > +
> > +     __release_region(&dax_region->res, range->start, range_len(range));
> > +     if (--dev_dax->nr_range == 0) {
> > +             kfree(dev_dax->ranges);
> > +             dev_dax->ranges = NULL;
> >       }
> > -     dev_dax->nr_range = 0;
> > +}
> > +
> > +static void free_dev_dax_ranges(struct dev_dax *dev_dax)
> > +{
> > +     while (dev_dax->nr_range)
> It's better to use READ_ONCE to get the value of dev_dax->nr_range,
> to prevent compiler optimization.

...only in the case where the compiler might try to turn this into an
infinite loop, but I don't think that can happen here outside of a
compiler bug. Usually READ_ONCE() is contending with SMP effects that
the compiler can't see.