Re: [PATCH 4/5] mm/swapfile: refcount block and queue before using blkcg_schedule_throttle()

[Ming Lei]

On Thu, Apr 16, 2020 at 06:25:32AM +0000, Luis Chamberlain wrote:
> On Thu, Apr 16, 2020 at 02:22:22PM +0800, Ming Lei wrote:
> > On Tue, Apr 14, 2020 at 04:19:01AM +0000, Luis Chamberlain wrote:
> > > block devices are refcounted so to ensure once its final user goes away it
> > > can be cleaned up by the lower layers properly. The block device's
> > > request_queue structure is also refcounted, however, if the last
> > > blk_put_queue() is called under atomic context the block layer has
> > > to defer removal.
> > > 
> > > By refcounting the block device during the use of blkcg_schedule_throttle(),
> > > we ensure ensure two things:
> > > 
> > > 1) the block device remains available during the call
> > > 2) we ensure avoid having to deal with the fact we're using the
> > >    request_queue structure in atomic context, since the last
> > >    blk_put_queue() will be called upon disk_release(), *after*
> > >    our own bdput().
> > > 
> > > This means this code path is *not* going to remove the request_queue
> > > structure, as we are ensuring some later upper layer disk_release()
> > > will be the one to release the request_queue structure for us.
> > > 
> > > Cc: Bart Van Assche <bvanassche@xxxxxxx>
> > > Cc: Omar Sandoval <osandov@xxxxxx>
> > > Cc: Hannes Reinecke <hare@xxxxxxxx>
> > > Cc: Nicolai Stange <nstange@xxxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > > Cc: yu kuai <yukuai3@xxxxxxxxxx>
> > > Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> > > ---
> > >  mm/swapfile.c | 14 ++++++++++++--
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index 6659ab563448..9285ff6030ca 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -3753,6 +3753,7 @@ static void free_swap_count_continuations(struct swap_info_struct *si)
> > >  void mem_cgroup_throttle_swaprate(struct mem_cgroup *memcg, int node,
> > >  				  gfp_t gfp_mask)
> > >  {
> > > +	struct block_device *bdev;
> > >  	struct swap_info_struct *si, *next;
> > >  	if (!(gfp_mask & __GFP_IO) || !memcg)
> > >  		return;
> > > @@ -3771,8 +3772,17 @@ void mem_cgroup_throttle_swaprate(struct mem_cgroup *memcg, int node,
> > >  	plist_for_each_entry_safe(si, next, &swap_avail_heads[node],
> > >  				  avail_lists[node]) {
> > >  		if (si->bdev) {
> > > -			blkcg_schedule_throttle(bdev_get_queue(si->bdev),
> > > -						true);
> > > +			bdev = bdgrab(si->bdev);
> > 
> > When swapon, the block_device has been opened in claim_swapfile(),
> > so no need to worry about the queue being gone here.
> 
> Thanks, so why bdev_get_queue() before?

bdev_get_queue() returns the request queue associated with the
the block device, and it is just that blkcg_schedule_throttle() needs
it.

Maybe I misunderstood your question, if yes, please explain it in
a bit detail.

Thanks,
Ming