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Message-ID: <20191113190952.GA4369@redhat.com>
Date: Wed, 13 Nov 2019 14:09:52 -0500
From: Jerome Glisse <jglisse@...hat.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Alex Williamson <alex.williamson@...hat.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Björn Töpel <bjorn.topel@...el.com>,
Christoph Hellwig <hch@...radead.org>,
Dan Williams <dan.j.williams@...el.com>,
Daniel Vetter <daniel@...ll.ch>,
Dave Chinner <david@...morbit.com>,
David Airlie <airlied@...ux.ie>,
"David S . Miller" <davem@...emloft.net>,
Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
Jonathan Corbet <corbet@....net>,
Magnus Karlsson <magnus.karlsson@...el.com>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Michal Hocko <mhocko@...e.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Paul Mackerras <paulus@...ba.org>,
Shuah Khan <shuah@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for
ZONE_DEVICE pages
On Tue, Nov 12, 2019 at 08:26:51PM -0800, John Hubbard wrote:
> An upcoming patch changes and complicates the refcounting and
> especially the "put page" aspects of it. In order to keep
> everything clean, refactor the devmap page release routines:
>
> * Rename put_devmap_managed_page() to page_is_devmap_managed(),
> and limit the functionality to "read only": return a bool,
> with no side effects.
>
> * Add a new routine, put_devmap_managed_page(), to handle checking
> what kind of page it is, and what kind of refcount handling it
> requires.
>
> * Rename __put_devmap_managed_page() to free_devmap_managed_page(),
> and limit the functionality to unconditionally freeing a devmap
> page.
>
> This is originally based on a separate patch by Ira Weiny, which
> applied to an early version of the put_user_page() experiments.
> Since then, Jérôme Glisse suggested the refactoring described above.
>
> Suggested-by: Jérôme Glisse <jglisse@...hat.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
Reviewed-by: Jérôme Glisse <jglisse@...hat.com>
> ---
> include/linux/mm.h | 27 ++++++++++++++++---
> mm/memremap.c | 67 ++++++++++++++++++++--------------------------
> 2 files changed, 53 insertions(+), 41 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a2adf95b3f9c..96228376139c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -967,9 +967,10 @@ static inline bool is_zone_device_page(const struct page *page)
> #endif
>
> #ifdef CONFIG_DEV_PAGEMAP_OPS
> -void __put_devmap_managed_page(struct page *page);
> +void free_devmap_managed_page(struct page *page);
> DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
> -static inline bool put_devmap_managed_page(struct page *page)
> +
> +static inline bool page_is_devmap_managed(struct page *page)
> {
> if (!static_branch_unlikely(&devmap_managed_key))
> return false;
> @@ -978,7 +979,6 @@ static inline bool put_devmap_managed_page(struct page *page)
> switch (page->pgmap->type) {
> case MEMORY_DEVICE_PRIVATE:
> case MEMORY_DEVICE_FS_DAX:
> - __put_devmap_managed_page(page);
> return true;
> default:
> break;
> @@ -986,6 +986,27 @@ static inline bool put_devmap_managed_page(struct page *page)
> return false;
> }
>
> +static inline bool put_devmap_managed_page(struct page *page)
> +{
> + bool is_devmap = page_is_devmap_managed(page);
> +
> + if (is_devmap) {
> + int count = page_ref_dec_return(page);
> +
> + /*
> + * devmap page refcounts are 1-based, rather than 0-based: if
> + * refcount is 1, then the page is free and the refcount is
> + * stable because nobody holds a reference on the page.
> + */
> + if (count == 1)
> + free_devmap_managed_page(page);
> + else if (!count)
> + __put_page(page);
> + }
> +
> + return is_devmap;
> +}
> +
> #else /* CONFIG_DEV_PAGEMAP_OPS */
> static inline bool put_devmap_managed_page(struct page *page)
> {
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 03ccbdfeb697..bc7e2a27d025 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -410,48 +410,39 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> EXPORT_SYMBOL_GPL(get_dev_pagemap);
>
> #ifdef CONFIG_DEV_PAGEMAP_OPS
> -void __put_devmap_managed_page(struct page *page)
> +void free_devmap_managed_page(struct page *page)
> {
> - int count = page_ref_dec_return(page);
> + /* Clear Active bit in case of parallel mark_page_accessed */
> + __ClearPageActive(page);
> + __ClearPageWaiters(page);
> +
> + mem_cgroup_uncharge(page);
>
> /*
> - * If refcount is 1 then page is freed and refcount is stable as nobody
> - * holds a reference on the page.
> + * When a device_private page is freed, the page->mapping field
> + * may still contain a (stale) mapping value. For example, the
> + * lower bits of page->mapping may still identify the page as
> + * an anonymous page. Ultimately, this entire field is just
> + * stale and wrong, and it will cause errors if not cleared.
> + * One example is:
> + *
> + * migrate_vma_pages()
> + * migrate_vma_insert_page()
> + * page_add_new_anon_rmap()
> + * __page_set_anon_rmap()
> + * ...checks page->mapping, via PageAnon(page) call,
> + * and incorrectly concludes that the page is an
> + * anonymous page. Therefore, it incorrectly,
> + * silently fails to set up the new anon rmap.
> + *
> + * For other types of ZONE_DEVICE pages, migration is either
> + * handled differently or not done at all, so there is no need
> + * to clear page->mapping.
> */
> - if (count == 1) {
> - /* Clear Active bit in case of parallel mark_page_accessed */
> - __ClearPageActive(page);
> - __ClearPageWaiters(page);
> -
> - mem_cgroup_uncharge(page);
> -
> - /*
> - * When a device_private page is freed, the page->mapping field
> - * may still contain a (stale) mapping value. For example, the
> - * lower bits of page->mapping may still identify the page as
> - * an anonymous page. Ultimately, this entire field is just
> - * stale and wrong, and it will cause errors if not cleared.
> - * One example is:
> - *
> - * migrate_vma_pages()
> - * migrate_vma_insert_page()
> - * page_add_new_anon_rmap()
> - * __page_set_anon_rmap()
> - * ...checks page->mapping, via PageAnon(page) call,
> - * and incorrectly concludes that the page is an
> - * anonymous page. Therefore, it incorrectly,
> - * silently fails to set up the new anon rmap.
> - *
> - * For other types of ZONE_DEVICE pages, migration is either
> - * 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))
> + page->mapping = NULL;
>
> - page->pgmap->ops->page_free(page);
> - } else if (!count)
> - __put_page(page);
> + page->pgmap->ops->page_free(page);
> }
> -EXPORT_SYMBOL(__put_devmap_managed_page);
> +EXPORT_SYMBOL(free_devmap_managed_page);
> #endif /* CONFIG_DEV_PAGEMAP_OPS */
> --
> 2.24.0
>
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