Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb

[David Hildenbrand <david@redhat.com>]

> Am 02.03.2020 um 21:07 schrieb Peter Xu <peterx@redhat.com>:
> 
> On Mon, Mar 02, 2020 at 08:02:34PM +0100, David Hildenbrand wrote:
>>> On 20.02.20 16:53, Peter Xu wrote:
>>> When follow_hugetlb_page() returns with *locked==0, it means we've got
>>> a VM_FAULT_RETRY within the fauling process and we've released the
>>> mmap_sem.  When that happens, we should stop and bail out.
>>> 
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>> mm/gup.c | 10 ++++++++++
>>> 1 file changed, 10 insertions(+)
>>> 
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 1b4411bd0042..76cb420c0fb7 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -849,6 +849,16 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>>>                i = follow_hugetlb_page(mm, vma, pages, vmas,
>>>                        &start, &nr_pages, i,
>>>                        gup_flags, locked);
>>> +                if (locked && *locked == 0) {
>>> +                    /*
>>> +                     * We've got a VM_FAULT_RETRY
>>> +                     * and we've lost mmap_sem.
>>> +                     * We must stop here.
>>> +                     */
>>> +                    BUG_ON(gup_flags & FOLL_NOWAIT);
>>> +                    BUG_ON(ret != 0);
>> 
>> Can we be sure ret is really set to != 0 at this point? At least,
>> reading the code this is not clear to me.
> 
> Here I wanted to make sure ret is zero (it's BUG_ON, not assert).

Sorry, I completely misread that BUG_ON for whatever reason, maybe I was staring for too long into my computer screen :)

> 
> "ret" is the fallback return value only if error happens when i==0.
> Here we want to make sure even if no page is pinned we'll return zero
> gracefully when VM_FAULT_RETRY happened when following the hugetlb
> pages.

Makes sense!

> 
>> 
>> Shouldn't we set "ret = i" and assert that i is an error (e.g., EBUSY?).
>> Or set -EBUSY explicitly?
> 
> No.  Here "i" could only be either positive (when we've got some pages
> pinned no matter where), or zero (when follow_hugetlb_page released
> the mmap_sem on the first page that it wants to pin).  So imo "i"
> should never be negative instead.

I briefly scanned the function and spotted some errors being returned, that‘s why I was wondering.

Thanks!