linux-kernel - Re: [PATCH] mm-move-mremap-from-file_operations-to-vm_operations

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Date:	Mon, 20 Jul 2015 20:31:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <js1304@...il.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]
	mm-move-mremap-from-file_operations-to-vm_operations_struct-fix

On 07/20, Benjamin LaHaise wrote:
>
> On Mon, Jul 20, 2015 at 07:33:11PM +0200, Oleg Nesterov wrote:
> >
> > And if we accept the fact this memory is locked and if we properly account
> > it, then may be we can just kill aio_migratepage(), aio_private_file(), and
> > change aio_setup_ring() to simply use install_special_mapping(). This will
> > greatly simplify the code. But let me remind that I know nothing about aio,
> > so please don't take my thoughts seriously.
>
> No, you can't get rid of that code.  The page migration is required when
> CPUs/memory is offlined and data needs to be moved to another node.

Of course, if we remove aio_migratepage() then aio can't be moved,

> Similarly, support for mremap() is also required for container migration /
> restoration.

This is not the problem. And one of the reasons to move ->mremap() into
vm_operations_struct was that install_special_mapping() can use it.

> Given how small the amount of memory aio can pin

I agree, but why should we worry about migration then? let this memory be
unmovable, don't use GFP_RECLAIMABLE/MOVABLE, etc.

But again, again, please ignore. This all is off-topic and my understanding
is very limited.

> it is unlikely that the accounting of
> how much aio has pinned will make any real difference in the big picture.

Agreed, but this can help to remove the system-wide aio-max-nr. Again,
unpriviliged user can steal aio.

Oleg.

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