linux-kernel - Re: [PATCH 2/4] bdi: Add bdi->id

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Date:   Sat, 3 Aug 2019 08:53:49 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     axboe@...nel.dk, jack@...e.cz, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, guro@...com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH 2/4] bdi: Add bdi->id

Hey, Matthew.

On Sat, Aug 03, 2019 at 08:39:08AM -0700, Matthew Wilcox wrote:
> On Sat, Aug 03, 2019 at 07:01:53AM -0700, Tejun Heo wrote:
> > There currently is no way to universally identify and lookup a bdi
> > without holding a reference and pointer to it.  This patch adds an
> > non-recycling bdi->id and implements bdi_get_by_id() which looks up
> > bdis by their ids.  This will be used by memcg foreign inode flushing.
> > 
> > I left bdi_list alone for simplicity and because while rb_tree does
> > support rcu assignment it doesn't seem to guarantee lossless walk when
> > walk is racing aginst tree rebalance operations.
> 
> This would seem like the perfect use for an allocating xarray.  That
> does guarantee lossless walk under the RCU lock.  You could get rid of the
> bdi_list too.

It definitely came to mind but there's a bunch of downsides to
recycling IDs or using radix tree for non-compacting allocations.

Thanks.

-- 
tejun