[2/3] mm/hmm: fix ZONE_DEVICE anon page mapping reuse
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Message ID 20190717001446.12351-3-rcampbell@nvidia.com
State New
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Series
  • mm/hmm: fixes for device private page migration
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Commit Message

Ralph Campbell July 17, 2019, 12:14 a.m. UTC
When a ZONE_DEVICE private page is freed, the page->mapping field can be
set. If this page is reused as an anonymous page, the previous value can
prevent the page from being inserted into the CPU's anon rmap table.
For example, when migrating a pte_none() page to device memory:
  migrate_vma(ops, vma, start, end, src, dst, private)
    migrate_vma_collect()
      src[] = MIGRATE_PFN_MIGRATE
    migrate_vma_prepare()
      /* no page to lock or isolate so OK */
    migrate_vma_unmap()
      /* no page to unmap so OK */
    ops->alloc_and_copy()
      /* driver allocates ZONE_DEVICE page for dst[] */
    migrate_vma_pages()
      migrate_vma_insert_page()
        page_add_new_anon_rmap()
          __page_set_anon_rmap()
            /* This check sees the page's stale mapping field */
            if (PageAnon(page))
              return
            /* page->mapping is not updated */

The result is that the migration appears to succeed but a subsequent CPU
fault will be unable to migrate the page back to system memory or worse.

Clear the page->mapping field when freeing the ZONE_DEVICE page so stale
pointer data doesn't affect future page use.

Fixes: b7a523109fb5c9d2d6dd ("mm: don't clear ->mapping in hmm_devmem_free")
Cc: stable@vger.kernel.org
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Jan Kara <jack@suse.cz>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
---
 kernel/memremap.c | 4 ++++
 1 file changed, 4 insertions(+)

Comments

John Hubbard July 17, 2019, 1:40 a.m. UTC | #1
On 7/16/19 5:14 PM, Ralph Campbell wrote:
> When a ZONE_DEVICE private page is freed, the page->mapping field can be
> set. If this page is reused as an anonymous page, the previous value can
> prevent the page from being inserted into the CPU's anon rmap table.
> For example, when migrating a pte_none() page to device memory:
>   migrate_vma(ops, vma, start, end, src, dst, private)
>     migrate_vma_collect()
>       src[] = MIGRATE_PFN_MIGRATE
>     migrate_vma_prepare()
>       /* no page to lock or isolate so OK */
>     migrate_vma_unmap()
>       /* no page to unmap so OK */
>     ops->alloc_and_copy()
>       /* driver allocates ZONE_DEVICE page for dst[] */
>     migrate_vma_pages()
>       migrate_vma_insert_page()
>         page_add_new_anon_rmap()
>           __page_set_anon_rmap()
>             /* This check sees the page's stale mapping field */
>             if (PageAnon(page))
>               return
>             /* page->mapping is not updated */
> 
> The result is that the migration appears to succeed but a subsequent CPU
> fault will be unable to migrate the page back to system memory or worse.
> 
> Clear the page->mapping field when freeing the ZONE_DEVICE page so stale
> pointer data doesn't affect future page use.
> 
> Fixes: b7a523109fb5c9d2d6dd ("mm: don't clear ->mapping in hmm_devmem_free")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Jan Kara <jack@suse.cz>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> ---
>  kernel/memremap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index bea6f887adad..238ae5d0ae8a 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -408,6 +408,10 @@ void __put_devmap_managed_page(struct page *page)
>  
>  		mem_cgroup_uncharge(page);
>  
> +		/* Clear anonymous page mapping to prevent stale pointers */

This is sufficiently complex, that some concise form of the documentation
that you've put in the commit description, needs to also exist right here, as
a comment. 

How's this read:

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 238ae5d0ae8a..e52e9da5d0a7 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -408,7 +408,27 @@ void __put_devmap_managed_page(struct page *page)
 
                mem_cgroup_uncharge(page);
 
-               /* Clear anonymous page mapping to prevent stale pointers */
+               /*
+                * 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 an 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;
 
?

thanks,

Patch
diff mbox series

diff --git a/kernel/memremap.c b/kernel/memremap.c
index bea6f887adad..238ae5d0ae8a 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -408,6 +408,10 @@  void __put_devmap_managed_page(struct page *page)
 
 		mem_cgroup_uncharge(page);
 
+		/* Clear anonymous page mapping to prevent stale pointers */
+		if (is_device_private_page(page))
+			page->mapping = NULL;
+
 		page->pgmap->ops->page_free(page);
 	} else if (!count)
 		__put_page(page);