John Hubbard <jhubbard@xxxxxxxxxx> writes:
> On 10/18/24 12:47 AM, David Hildenbrand wrote:
>> On 18.10.24 03:17, John Hubbard wrote:
[...]
> And actually this whole thing of "pin the pages, just for a short time, even
> though you're not allowed to" is partly why this area is so entertaining.
I'm looking at your v3 but as an aside I disagree with this
statement. AFAIK you're always allowed to pin the pages for a short time
(ie. !FOLL_LONGTERM), or did I misunderstand your comment?
>> diff --git a/mm/gup.c b/mm/gup.c
>> index a82890b46a36..81fc8314e687 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -2403,8 +2403,9 @@ static int migrate_longterm_unpinnable_folios(
>> * -EAGAIN. The caller should re-pin the entire range with
>> FOLL_PIN and then
>> * call this routine again.
>> *
>> - * If an error other than -EAGAIN occurs, this indicates a
>> migration failure.
>> - * The caller should give up, and propagate the error back up the
>> call stack.
>> + * If an error occurs, all folios are unpinned. If an error other than
>> + * -EAGAIN occurs, this indicates a migration failure. The caller
>> should give
>> + * up, and propagate the error back up the call stack.
>> *
>> * If everything is OK and all folios in the range are allowed to
>> be pinned,
>> * then this routine leaves all folios pinned and returns zero for
>> success.
>> @@ -2437,8 +2438,10 @@ static long
>> check_and_migrate_movable_pages(unsigned long nr_pages,
>> long i, ret;
>> folios = kmalloc_array(nr_pages, sizeof(*folios),
>> GFP_KERNEL);
>> - if (!folios)
>> + if (!folios) {
>> + unpin_user_pages(pages, nr_pages);
>> return -ENOMEM;
>> + }
>> for (i = 0; i < nr_pages; i++)
>> folios[i] = page_folio(pages[i]);
>> Then, check_and_migrate_movable_pages() will never return with an
>> error and
>> having folios pinned.
>> If check_and_migrate_movable_pages() ->
>> check_and_migrate_movable_folios()
>> returns "0", all folios remain pinned an no harm is done.
>> Consequently, I think patch #2 is not really required, because it
>> doesn't
>> perform the temporary allocation that could fail with -ENOMEM.
>>
>
> Yes!
>
>> Sorry for taking a closer look only now ...
>>
>
> It's all still in review, so the timing is perfectly fine. I really
> appreciate the closer look, it's definitely making things better.
>
>
> thanks,
