Re: [PATCH v3 2/4] mm/gup: clean up follow_pfn_pte() slightly

John Hubbard
2022-02-03T21:19:41Z

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