linux-kernel - Re: [RFC 00/10] implement alternative and much simpler id allocator

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Date:   Mon, 12 Dec 2016 13:05:01 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "kent.overstreet@...il.com" <kent.overstreet@...il.com>
Subject: Re: [RFC 00/10] implement alternative and much simpler id allocator

Hello, Matthew.

On Mon, Dec 12, 2016 at 05:35:17PM +0000, Matthew Wilcox wrote:
> I know the preload followed by preload_end looks wrong.  I don't
> think it's broken though.  If we get preempted, then the worst
> situation is that we'll end up with the memory we preallocated being
> allocated to somebody else.  Then the attempt to allocate memory can
> fail, and we'll return -EAGAIN, at which point all callers are
> supposed to return to the pre_get() stage.  Certainly that's what
> ida_simple_get() does.

Ah, right, ida_pre_get() doesn't have any protection against other
task allocating inbetween pre_get and the actual allocation, so it
should retry on failure.  Yeah, then the proposed preloading wouldn't
be wrong.  It'd be nice to explain what's going on tho.

> I'd definitely be open to changing the IDA API.  I know Kent had
> some thoughts on that including splitting the simple lock into a
> per-IDA lock.  His last work on it was here, I believe:
> 
> https://evilpiepirate.org/git/linux-bcache.git/log/?h=idr

Yeah, that was a big re-write, but for now I think it'd be nice to
replace ida's pre_get mechanism with something similar to idr's
preload so that they're more consistent.  There aren't that many
direct users of ida_pre_get(), so it shouldn't be too difficult to
change.

Thanks.

-- 
tejun