Re: [PATCH] mm: Cleanup __put_devmap_managed_page() vs ->page_free()

From: Dan Williams <dan.j.williams@intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: "John Hubbard" <jhubbard@nvidia.com>, "Jan Kara" <jack@suse.cz>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Linux MM" <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: Cleanup __put_devmap_managed_page() vs ->page_free()
Date: Wed, 13 Nov 2019 23:25:23 -0800	[thread overview]
Message-ID: <CAPcyv4iyD7f-3Ws7HpNvfNwO52CK7W-iF7Vsxv3MrGWzALsMGA@mail.gmail.com> (raw)
In-Reply-To: <20191114071903.GA26307@lst.de>

On Wed, Nov 13, 2019 at 11:19 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Wed, Nov 13, 2019 at 04:07:22PM -0800, Dan Williams wrote:
> >  static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
> >  {
> > -     if (!pgmap->ops || !pgmap->ops->page_free) {
> > +     if (!pgmap->ops || (pgmap->type == MEMORY_DEVICE_PRIVATE
> > +                             && !pgmap->ops->page_free)) {
>
> I don't think this check is correct.  You only want the the ops null check
> or MEMORY_DEVICE_PRIVATE as well now, i.e.:
>
>         if (pgmap->type == MEMORY_DEVICE_PRIVATE &&
>             (!pgmap->ops || !pgmap->ops->page_free)) {
>
> > @@ -476,10 +471,17 @@ void __put_devmap_managed_page(struct page *page)
> >                * handled differently or not done at all, so there is no need
> >                * to clear page->mapping.
> >                */
> > -             if (is_device_private_page(page))
> > -                     page->mapping = NULL;
> > +             if (is_device_private_page(page)) {
> > +                     /* Clear Active bit in case of parallel mark_page_accessed */
>
> This adds a > 80 char line.  But that whole flow of the function seems
> rather odd now.
>
> Why can't we do:
>
>         if (count == 0) {
>                 __put_page(page);
>         } else if (is_device_private_page(page)) {
>                 __ClearPageActive(page);
>                 __ClearPageWaiters(page);
>
>                 mem_cgroup_uncharge(page);
>                 page->mapping = NULL;
>                 page->pgmap->ops->page_free(page);
>         } else {
>                 wake_up_var(&page->_refcount);
>         }
>

All the above looks good to me will spin a v2.

> (except for the fact that I don't get the point of calling __put_page
> on a refcount of zero, but that is separate from this patch).

That looked odd to me as well until I recalled that we did that to
simplify the pgmap reference counting.

71389703839e mm, zone_device: Replace {get, put}_zone_device_page()
with a single reference to fix pmem crash

I'll add a comment in v2.
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