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Date:   Thu, 21 Mar 2019 16:24:46 -0400
From:   Jerome Glisse <jglisse@redhat.com>
To:     Thomas Hellstrom <thellstrom@vmware.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Subject: Re: [RFC PATCH RESEND 2/3] mm: Add an apply_to_pfn_range interface
Message-ID: <20190321202445.GA15074@redhat.com>
References: <20190321132140.114878-1-thellstrom@vmware.com>
 <20190321132140.114878-3-thellstrom@vmware.com>
 <20190321135202.GC2904@redhat.com>
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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,
J?r?me