Re: [RFC PATCH RESEND 2/3] mm: Add an apply_to_pfn_range interface

From: Jerome Glisse <jglisse@redhat.com>
To: Thomas Hellstrom <thellstrom@vmware.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"willy@infradead.org" <willy@infradead.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"jrdr.linux@gmail.com" <jrdr.linux@gmail.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"minchan@kernel.org" <minchan@kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	Linux-graphics-maintainer <Linux-graphics-maintainer@vmware.com>,
	"mhocko@suse.com" <mhocko@suse.com>,
	"ying.huang@intel.com" <ying.huang@intel.com>,
	"riel@surriel.com" <riel@surriel.com>
Subject: Re: [RFC PATCH RESEND 2/3] mm: Add an apply_to_pfn_range interface
Date: Thu, 21 Mar 2019 16:24:46 -0400	[thread overview]
Message-ID: <20190321202445.GA15074@redhat.com> (raw)
In-Reply-To: <c9d05087a0fb9002145aa2f7c58552615a694e9e.camel@vmware.com>

On Thu, Mar 21, 2019 at 07:59:35PM +0000, Thomas Hellstrom wrote:
> On Thu, 2019-03-21 at 09:52 -0400, Jerome Glisse wrote:
> > On Thu, Mar 21, 2019 at 01:22:35PM +0000, Thomas Hellstrom wrote:
> > > This is basically apply_to_page_range with added functionality:
> > > Allocating missing parts of the page table becomes optional, which
> > > means that the function can be guaranteed not to error if
> > > allocation
> > > is disabled. Also passing of the closure struct and callback
> > > function
> > > becomes different and more in line with how things are done
> > > elsewhere.
> > > 
> > > Finally we keep apply_to_page_range as a wrapper around
> > > apply_to_pfn_range
> > 
> > The apply_to_page_range() is dangerous API it does not follow other
> > mm patterns like mmu notifier. It is suppose to be use in arch code
> > or vmalloc or similar thing but not in regular driver code. I see
> > it has crept out of this and is being use by few device driver. I am
> > not sure we should encourage that.
> 
> I can certainly remove the EXPORT of the new apply_to_pfn_range() which
> will make sure its use stays within the mm code. I don't expect any
> additional usage except for the two address-space utilities.
> 
> I'm looking for examples to see how it could be more in line with the
> rest of the mm code. The main difference from the pattern in, for
> example, page_mkclean() seems to be that it's lacking the
> mmu_notifier_invalidate_start() and mmu_notifier_invalidate_end()?
> Perhaps the intention is to have the pte leaf functions notify on pte
> updates? How does this relate to arch_enter_lazy_mmu() which is called
> outside of the page table locks? The documentation appears a bit
> scarce...

Best is to use something like walk_page_range() and have proper mmu
notifier in the callback. The apply_to_page_range() is broken for
huge page (THP) and other things like that. Thought you should not
have THP within mmap of a device file (at least i do not thing any
driver does that).

Cheers,
Jérôme