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From: Dan Williams <dan.j.williams@intel.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Christoph Hellwig <hch@lst.de>,
	Shiyang Ruan <ruansy.fnst@fujitsu.com>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	david <david@fromorbit.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path
Date: Thu, 18 Mar 2021 10:03:06 -0700	[thread overview]
Message-ID: <CAPcyv4g8=kGoQiY14CDEZryb-7T1_tePnC_-21w-wTfA7fQcDg@mail.gmail.com> (raw)
In-Reply-To: <66514812-6a24-8e2e-7be5-c61e188fecc4@oracle.com>

On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 3/18/21 4:08 AM, Dan Williams wrote:
> > Now that device-dax and filesystem-dax are guaranteed to unmap all user
> > mappings of devmap / DAX pages before tearing down the 'struct page'
> > array, get_user_pages_fast() can rely on its traditional synchronization
> > method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> > device shutdown. Specifically the unmap guarantee ensures that gup-fast
> > either succeeds in taking a page reference (lock-less), or it detects a
> > need to fall back to the slow path where the device presence can be
> > revalidated with locks held.
>
> [...]
>
> > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
> >
> >  #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > +
> >  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> >                            unsigned long end, unsigned int flags,
> >                            struct page **pages, int *nr)
> >  {
> >       int nr_start = *nr;
> > -     struct dev_pagemap *pgmap = NULL;
> >
> >       do {
> > -             struct page *page = pfn_to_page(pfn);
> > +             struct page *page;
> > +
> > +             /*
> > +              * Typically pfn_to_page() on a devmap pfn is not safe
> > +              * without holding a live reference on the hosting
> > +              * pgmap. In the gup-fast path it is safe because any
> > +              * races will be resolved by either gup-fast taking a
> > +              * reference or the shutdown path unmapping the pte to
> > +              * trigger gup-fast to fall back to the slow path.
> > +              */
> > +             page = pfn_to_page(pfn);
> >
> > -             pgmap = get_dev_pagemap(pfn, pgmap);
> > -             if (unlikely(!pgmap)) {
> > -                     undo_dev_pagemap(nr, nr_start, flags, pages);
> > -                     return 0;
> > -             }
> >               SetPageReferenced(page);
> >               pages[*nr] = page;
> >               if (unlikely(!try_grab_page(page, flags))) {
>
> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after
> try_grab_page() for checking pgmap type to see if we are in a device-dax
> longterm pin?
>

Yes. I still need to answer the question of whether mapping
invalidation triggers longterm pin holders to relinquish their hold,
but that's a problem regardless of whether gup-fast is supported or
not.
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WARNING: multiple messages have this Message-ID
From: Dan Williams <dan.j.williams@intel.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Christoph Hellwig <hch@lst.de>,
	Shiyang Ruan <ruansy.fnst@fujitsu.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	david <david@fromorbit.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path
Date: Thu, 18 Mar 2021 10:03:06 -0700	[thread overview]
Message-ID: <CAPcyv4g8=kGoQiY14CDEZryb-7T1_tePnC_-21w-wTfA7fQcDg@mail.gmail.com> (raw)
In-Reply-To: <66514812-6a24-8e2e-7be5-c61e188fecc4@oracle.com>

On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 3/18/21 4:08 AM, Dan Williams wrote:
> > Now that device-dax and filesystem-dax are guaranteed to unmap all user
> > mappings of devmap / DAX pages before tearing down the 'struct page'
> > array, get_user_pages_fast() can rely on its traditional synchronization
> > method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> > device shutdown. Specifically the unmap guarantee ensures that gup-fast
> > either succeeds in taking a page reference (lock-less), or it detects a
> > need to fall back to the slow path where the device presence can be
> > revalidated with locks held.
>
> [...]
>
> > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
> >
> >  #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > +
> >  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> >                            unsigned long end, unsigned int flags,
> >                            struct page **pages, int *nr)
> >  {
> >       int nr_start = *nr;
> > -     struct dev_pagemap *pgmap = NULL;
> >
> >       do {
> > -             struct page *page = pfn_to_page(pfn);
> > +             struct page *page;
> > +
> > +             /*
> > +              * Typically pfn_to_page() on a devmap pfn is not safe
> > +              * without holding a live reference on the hosting
> > +              * pgmap. In the gup-fast path it is safe because any
> > +              * races will be resolved by either gup-fast taking a
> > +              * reference or the shutdown path unmapping the pte to
> > +              * trigger gup-fast to fall back to the slow path.
> > +              */
> > +             page = pfn_to_page(pfn);
> >
> > -             pgmap = get_dev_pagemap(pfn, pgmap);
> > -             if (unlikely(!pgmap)) {
> > -                     undo_dev_pagemap(nr, nr_start, flags, pages);
> > -                     return 0;
> > -             }
> >               SetPageReferenced(page);
> >               pages[*nr] = page;
> >               if (unlikely(!try_grab_page(page, flags))) {
>
> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after
> try_grab_page() for checking pgmap type to see if we are in a device-dax
> longterm pin?
>

Yes. I still need to answer the question of whether mapping
invalidation triggers longterm pin holders to relinquish their hold,
but that's a problem regardless of whether gup-fast is supported or
not.

WARNING: multiple messages have this Message-ID
From: Dan Williams <dan.j.williams@intel.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Christoph Hellwig <hch@lst.de>,
	Shiyang Ruan <ruansy.fnst@fujitsu.com>,
	 Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	 Ira Weiny <ira.weiny@intel.com>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	 Andrew Morton <akpm@linux-foundation.org>,
	david <david@fromorbit.com>,
	 linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	 linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path
Date: Thu, 18 Mar 2021 10:03:06 -0700	[thread overview]
Message-ID: <CAPcyv4g8=kGoQiY14CDEZryb-7T1_tePnC_-21w-wTfA7fQcDg@mail.gmail.com> (raw)
In-Reply-To: <66514812-6a24-8e2e-7be5-c61e188fecc4@oracle.com>

On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 3/18/21 4:08 AM, Dan Williams wrote:
> > Now that device-dax and filesystem-dax are guaranteed to unmap all user
> > mappings of devmap / DAX pages before tearing down the 'struct page'
> > array, get_user_pages_fast() can rely on its traditional synchronization
> > method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> > device shutdown. Specifically the unmap guarantee ensures that gup-fast
> > either succeeds in taking a page reference (lock-less), or it detects a
> > need to fall back to the slow path where the device presence can be
> > revalidated with locks held.
>
> [...]
>
> > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
> >
> >  #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > +
> >  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> >                            unsigned long end, unsigned int flags,
> >                            struct page **pages, int *nr)
> >  {
> >       int nr_start = *nr;
> > -     struct dev_pagemap *pgmap = NULL;
> >
> >       do {
> > -             struct page *page = pfn_to_page(pfn);
> > +             struct page *page;
> > +
> > +             /*
> > +              * Typically pfn_to_page() on a devmap pfn is not safe
> > +              * without holding a live reference on the hosting
> > +              * pgmap. In the gup-fast path it is safe because any
> > +              * races will be resolved by either gup-fast taking a
> > +              * reference or the shutdown path unmapping the pte to
> > +              * trigger gup-fast to fall back to the slow path.
> > +              */
> > +             page = pfn_to_page(pfn);
> >
> > -             pgmap = get_dev_pagemap(pfn, pgmap);
> > -             if (unlikely(!pgmap)) {
> > -                     undo_dev_pagemap(nr, nr_start, flags, pages);
> > -                     return 0;
> > -             }
> >               SetPageReferenced(page);
> >               pages[*nr] = page;
> >               if (unlikely(!try_grab_page(page, flags))) {
>
> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after
> try_grab_page() for checking pgmap type to see if we are in a device-dax
> longterm pin?
>

Yes. I still need to answer the question of whether mapping
invalidation triggers longterm pin holders to relinquish their hold,
but that's a problem regardless of whether gup-fast is supported or
not.


  reply	other threads:[~2021-03-18 17:03 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-18  4:08 [PATCH 0/3] mm, pmem: Force unmap pmem on surprise remove Dan Williams
2021-03-18  4:08 ` Dan Williams
2021-03-18  4:08 ` [PATCH 1/3] mm/memory-failure: Prepare for mass memory_failure() Dan Williams
2021-03-18  4:08   ` Dan Williams
2021-03-18  4:08 ` [PATCH 2/3] mm, dax, pmem: Introduce dev_pagemap_failure() Dan Williams
2021-03-18  4:08   ` Dan Williams
2021-03-18  4:45   ` Darrick J. Wong
2021-03-18  4:45     ` Darrick J. Wong
2021-03-18 19:26     ` Dan Williams
2021-03-18 19:26       ` Dan Williams
2021-03-18 19:26       ` Dan Williams
2021-03-18  4:57   ` Dave Chinner
2021-03-18  4:57     ` Dave Chinner
2021-03-18 19:20     ` Dan Williams
2021-03-18 19:20       ` Dan Williams
2021-03-18 19:20       ` Dan Williams
2021-03-20  1:46       ` Dave Chinner
2021-03-20  1:46         ` Dave Chinner
2021-03-20  2:39         ` Dan Williams
2021-03-20  2:39           ` Dan Williams
2021-03-20  2:39           ` Dan Williams
2021-03-18  4:08 ` [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path Dan Williams
2021-03-18  4:08   ` Dan Williams
2021-03-18 10:00   ` Joao Martins
2021-03-18 10:00     ` Joao Martins
2021-03-18 17:03     ` Dan Williams [this message]
2021-03-18 17:03       ` Dan Williams
2021-03-18 17:03       ` Dan Williams
2021-03-25 14:34       ` Jason Gunthorpe
2021-03-29 23:24         ` Dan Williams
2021-03-29 23:24           ` Dan Williams
2021-03-29 23:24           ` Dan Williams
2021-03-30 13:49           ` Jason Gunthorpe
2021-03-24 17:45     ` Dan Williams
2021-03-24 17:45       ` Dan Williams
2021-03-24 17:45       ` Dan Williams
2021-03-24 19:00       ` Joao Martins
2021-03-24 19:00         ` Joao Martins
2021-04-01 19:54         ` Joao Martins
2021-04-01 19:54           ` Joao Martins
2021-03-25 14:37   ` Jason Gunthorpe
2021-03-25 13:02 ` [PATCH 0/3] mm, pmem: Force unmap pmem on surprise remove David Hildenbrand
2021-03-25 13:02   ` David Hildenbrand

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