Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference

From: Jerome Glisse <jglisse@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	linux-kernel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Logan Gunthorpe <logang@deltatee.com>,
	Kirill Shutemov <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference
Date: Fri, 28 Apr 2017 15:16:11 -0400 (EDT)	[thread overview]
Message-ID: <1295710462.4327805.1493406971970.JavaMail.zimbra@redhat.com> (raw)
In-Reply-To: <CAPcyv4hvBKG8t3e3QvUnmkaopeM8eTniz5JPVkrZ5Puu5eaViw@mail.gmail.com>

> On Fri, Apr 28, 2017 at 11:00 AM, Jerome Glisse <jglisse@redhat.com> wrote:
> >> On Fri, Apr 28, 2017 at 10:34 AM, Jerome Glisse <jglisse@redhat.com>
> >> wrote:
> >> >> Kirill points out that the calls to {get,put}_dev_pagemap() can be
> >> >> removed from the mm fast path if we take a single get_dev_pagemap()
> >> >> reference to signify that the page is alive and use the final put of
> >> >> the
> >> >> page to drop that reference.
> >> >>
> >> >> This does require some care to make sure that any waits for the
> >> >> percpu_ref to drop to zero occur *after* devm_memremap_page_release(),
> >> >> since it now maintains its own elevated reference.
> >> >
> >> > This is NAK from HMM point of view as i need those call. So if you
> >> > remove
> >> > them now i will need to add them back as part of HMM.
> >>
> >> I thought you only need them at page free time? You can still hook
> >> __put_page().
> >
> > No, i need a hook when page refcount reach 1, not 0. That being said
> > i don't care about put_dev_pagemap(page->pgmap); so that part of the
> > patch is fine from HMM point of view but i definitly need to hook my-
> > self in the general put_page() function.
> >
> > So i will have to undo part of this patch for HMM (put_page() will
> > need to handle ZONE_DEVICE page differently).
> 
> Ok, I'd rather this go in now since it fixes the existing use case,
> and unblocks the get_user_pages_fast() conversion to generic code.
> That also gives Kirill and -mm folks a chance to review what HMM wants
> to do on top of the page_ref infrastructure.  The
> {get,put}_zone_device_page interface went in in 4.5 right before
> page_ref went in during 4.6, so it was just an oversight that
> {get,put}_zone_device_page were not removed earlier.
> 

I don't mind this going in, i am hopping people won't ignore HMM patchset
once i repost after 4.12 merge window. Note that there is absolutely no way
around me hooking up inside put_page(). The only other way to do it would
be to modify virtualy all places that call that function to handle ZONE_DEVICE
case.

Jérôme