linux-kernel - Re: [WiP]: aio support for migrating pages (Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non

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Date:	Fri, 17 May 2013 14:30:03 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Zach Brown <zab@...hat.com>
Cc:	Tang Chen <tangchen@...fujitsu.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan@...nel.org>,
	Lin Feng <linfeng@...fujitsu.com>, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, khlebnikov@...nvz.org, walken@...gle.com,
	kamezawa.hiroyu@...fujitsu.com, riel@...hat.com,
	rientjes@...gle.com, isimatu.yasuaki@...fujitsu.com,
	wency@...fujitsu.com, laijs@...fujitsu.com, jiang.liu@...wei.com,
	jmoyer@...hat.com, linux-mm@...ck.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [WiP]: aio support for migrating pages (Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non_movable())

On Fri, May 17, 2013 at 11:17:08AM -0700, Zach Brown wrote:
> > I ended up working on this a bit today, and managed to cobble together 
> > something that somewhat works -- please see the patch below.
> 
> Just some quick observations:
> 
> > +	ctx->ctx_file = anon_inode_getfile("[aio]", &aio_ctx_fops, ctx, O_RDWR);
> > +	if (IS_ERR(ctx->ctx_file)) {
> > +		ctx->ctx_file = NULL;
> > +		return -EAGAIN;
> > +	}
> 
> It's too bad that aio contexts will now be accounted against the filp
> limits (get_empty_filp -> files_stat.max_files, etc). 

Yeah, that is a downside of this approach.  It would be possible to to 
do it with only an inode/address_space, but that would mean bypassing 
do_mmap(), which is not worth considering.  If it is really an issue, we 
could add a flag to bypass that limit since aio has its own.  
anon_inode_getfile() as it stands is a major problem.

> > +	for (i=0; i<nr_pages; i++) {
> > +		struct page *page;
> > +		void *ptr;
> > +		page = find_or_create_page(ctx->ctx_file->f_inode->i_mapping,
> > +					   i, GFP_KERNEL);
> > +		if (!page) {
> > +			break;
> > +		}
> > +		ptr = kmap(page);
> > +		clear_page(ptr);
> > +		kunmap(page);
> > +		SetPageUptodate(page);
> > +		SetPageDirty(page);
> > +		unlock_page(page);
> > +	}
> 
> If they're GFP_KERNEL then you don't need to kmap them.  But we probably
> want to allocate with GFP_HIGHUSER and then use clear_user_highpage() to
> zero them?

Adding __GFP_ZERO would fix that too. The next respin will include that 
change.  I also have to properly handle the mremap() case as well.

		-ben
-- 
"Thought is the essence of where you are now."
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