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Date: Fri, 17 May 2013 14:30:03 -0400
From: Benjamin LaHaise <bcrl@kvack.org>
To: Zach Brown <zab@redhat.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>, Mel Gorman <mgorman@suse.de>,
        Minchan Kim <minchan@kernel.org>, Lin Feng <linfeng@cn.fujitsu.com>,
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        Marek Szyprowski <m.szyprowski@samsung.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())
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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."