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Date:   Thu, 3 Feb 2022 13:19:33 -0800
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Subject: Re: [PATCH v3 2/4] mm/gup: clean up follow_pfn_pte() slightly
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To:     Matthew Wilcox <willy@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2/3/22 07:18, Matthew Wilcox wrote:
...
>>> This will also modify the error code returned from follow_page().
>>
>> Er, but isn't that the whole point of this entire design? It is what
>> the commit that added it says:
>>
>> commit 1027e4436b6a5c413c95d95e50d0f26348a602ac
>> Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> Date:   Fri Sep 4 15:47:55 2015 -0700
>>
>>      mm: make GUP handle pfn mapping unless FOLL_GET is requested
>>      
>>      With DAX, pfn mapping becoming more common.  The patch adjusts GUP code to
>>      cover pfn mapping for cases when we don't need struct page to proceed.
>>      
>>      To make it possible, let's change follow_page() code to return -EEXIST
>>      error code if proper page table entry exists, but no corresponding struct
>>      page.  __get_user_page() would ignore the error code and move to the next
>>      page frame.
>>      
>>      The immediate effect of the change is working MAP_POPULATE and mlock() on
>>      DAX mappings.
>>
>>> A quick audit shows that at least the user in mm/migrate.c will
>>> propagate this error code to userspace and I'm not sure the change
>>> in error code will not break something... EEXIST is a bit strange
>>> error code to get from move_pages(2).
>>
>> That makes sense, maybe move_pages should squash the return codes to
>> EEXIST?
> 
> I think EFAULT is the closest:
>                This  is  a  zero  page  or the memory area is not mapped by the
>                process.
> 
> EBUSY implies it can be tried again later.
> 

OK. I definitely need to rework the commit description now, but the diffs are
looking like this:

diff --git a/mm/gup.c b/mm/gup.c
index 65575ae3602f..cad3f28492e3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -439,10 +439,6 @@ static struct page *no_page_table(struct vm_area_struct *vma,
  static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  		pte_t *pte, unsigned int flags)
  {
-	/* No page to get reference */
-	if (flags & (FOLL_GET | FOLL_PIN))
-		return -EFAULT;
-
  	if (flags & FOLL_TOUCH) {
  		pte_t entry = *pte;

@@ -1180,8 +1176,13 @@ static long __get_user_pages(struct mm_struct *mm,
  		} else if (PTR_ERR(page) == -EEXIST) {
  			/*
  			 * Proper page table entry exists, but no corresponding
-			 * struct page.
+			 * struct page. If the caller expects **pages to be
+			 * filled in, bail out now, because that can't be done
+			 * for this page.
  			 */
+			if (pages)
+				goto out;
+
  			goto next_page;
  		} else if (IS_ERR(page)) {
  			ret = PTR_ERR(page);
diff --git a/mm/migrate.c b/mm/migrate.c
index c7da064b4781..be0d5ae36dc1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1761,6 +1761,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
  			continue;
  		}

+		/*
+		 * The move_pages() man page does not have an -EEXIST choice, so
+		 * use -EFAULT instead.
+		 */
+		if (err == -EEXIST)
+			err = -EFAULT;
+
  		/*
  		 * If the page is already on the target node (!err), store the
  		 * node, otherwise, store the err.

thanks,
-- 
John Hubbard
NVIDIA