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Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
From:   Jens Axboe <axboe@kernel.dk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        fsdevel <linux-fsdevel@vger.kernel.org>, linux-aio@kvack.org,
        linux-mm <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dan Carpenter <dan.carpenter@oracle.com>
References: <20181128183531.5139-1-willy@infradead.org>
 <09e3d156-66fc-ca17-efac-63f080a27a1d@kernel.dk>
 <20181211184553.GH6830@bombadil.infradead.org>
 <75267003-9407-101f-33ee-685e345a2c8a@kernel.dk>
Message-ID: <e4822d16-efec-46f1-8f4c-ed68b8e5f27f@kernel.dk>
Date:   Tue, 11 Dec 2018 11:53:36 -0700
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On 12/11/18 11:46 AM, Jens Axboe wrote:
> On 12/11/18 11:45 AM, Matthew Wilcox wrote:
>> On Tue, Dec 11, 2018 at 11:41:55AM -0700, Jens Axboe wrote:
>>> On Wed, Nov 28, 2018 at 11:35 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>>
>>>>         rcu_read_lock();
>>>> -       table = rcu_dereference(mm->ioctx_table);
>>>> -
>>>> -       if (!table || id >= table->nr)
>>>> -               goto out;
>>>> -
>>>> -       ctx = rcu_dereference(table->table[id]);
>>>> +       ctx = xa_load(&mm->ioctx, id);
>>>>         if (ctx && ctx->user_id == ctx_id) {
>>>>                 if (percpu_ref_tryget_live(&ctx->users))
>>>>                         ret = ctx;
>>>>         }
>>>
>>> Question on this part - do we need that RCU read lock around this now? I
>>> don't think we do.
>>
>> I think we need the rcu read lock here to prevent ctx from being freed
>> under us by free_ioctx().
> 
> Then that begs the question, how about __xa_load() that is already called
> under RCU read lock?

Something like this, mem remap has an existing user that can use this
too already.

-- 
Jens Axboe