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Date: Tue, 14 Apr 2020 08:44:47 -0700 From: Christoph Hellwig <hch@...radead.org> To: Luis Chamberlain <mcgrof@...nel.org> Cc: axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org, gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz, ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Omar Sandoval <osandov@...com>, Hannes Reinecke <hare@...e.com>, Michal Hocko <mhocko@...nel.org> Subject: Re: [PATCH 4/5] mm/swapfile: refcount block and queue before using blkcg_schedule_throttle() 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@....org> > Cc: Omar Sandoval <osandov@...com> > Cc: Hannes Reinecke <hare@...e.com> > Cc: Nicolai Stange <nstange@...e.de> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org> > Cc: Michal Hocko <mhocko@...nel.org> > Cc: yu kuai <yukuai3@...wei.com> > Signed-off-by: Luis Chamberlain <mcgrof@...nel.org> > --- > 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); > + if (!bdev) > + continue; > + /* > + * By adding our own bdgrab() we ensure the queue > + * sticks around until disk_release(), and so we ensure > + * our release of the request_queue does not happen in > + * atomic context. > + */ > + blkcg_schedule_throttle(bdev_get_queue(bdev), true); > + bdput(bdev); I don't understand the atomic part of the comment. How does bdgrab/bdput help us there?
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