Message-ID: <53816439-6473-1c4f-2134-02cd1c46cfe8@kernel.dk>
Date:   Tue, 7 Feb 2023 10:05:35 -0700
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Subject: Re: [PATCH 09/19] io_uring: convert to use vm_account
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To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        jhubbard@nvidia.com, tjmercier@google.com, hannes@cmpxchg.org,
        surenb@google.com, mkoutny@suse.com, daniel@ffwll.ch,
        "Daniel P . Berrange" <berrange@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org
References: <cover.c238416f0e82377b449846dbb2459ae9d7030c8e.1675669136.git-series.apopple@nvidia.com>
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 <52d41a7e-1407-e74f-9206-6dd583b7b6b5@kernel.dk> <87k00unusm.fsf@nvidia.com>
 <eff3cc48-7279-2fbf-fdbd-f35eff2124d0@kernel.dk>
 <Y+JmdMJhPEGN0Zw+@nvidia.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <Y+JmdMJhPEGN0Zw+@nvidia.com>
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On 2/7/23 7:55?AM, Jason Gunthorpe wrote:
> On Tue, Feb 07, 2023 at 07:28:56AM -0700, Jens Axboe wrote:
> 
>> Outside of that, we're now doubling the amount of memory associated with
>> tracking this. That isn't necessarily a showstopper, but it is not
>> ideal. I didn't take a look at the other conversions (again, because
>> they were not sent to me), but seems like the task_struct and flags
>> could just be passed in as they may very well be known to many/most
>> callers?
> 
> For places doing the mm accounting type it cannot use the task struct
> as the underlying mm can be replaced and keep the task, IIRC.
> 
> We just had a bug in VFIO related to this..
> 
> If we could go back from the mm to the task (even a from a destroyed
> mm though) that might work to reduce storage?

Then maybe just nest them:

struct small_one {
	struct mm_struct *mm;
	struct user_struct *user;
};

struct big_one {
	struct small_one foo;
	struct task_struct *task;
	enum vm_account_flags flags;
};

and have the real helpers deal with small_one, and wrappers around that
taking big_one that just passes in the missing bits. Then users that
don't need the extra bits can just use the right API.

-- 
Jens Axboe