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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20190803161743.GB932@bombadil.infradead.org>
Date:   Sat, 3 Aug 2019 09:17:43 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Tejun Heo <tj@...nel.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

On Sat, Aug 03, 2019 at 08:53:49AM -0700, Tejun Heo wrote:
> 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.

Ah, I wasn't sure what would happen if you recycled an ID.  I agree, the
radix tree is pretty horrid for monotonically increasing IDs.  I'm still
working on the maple tree to replace it, but that's going slower than
I would like, so I can't in good conscience ask you to wait for it to
be ready.