Re: [PATCH v2] mm/page_alloc: Return nr_populated when the array is full

[Chuck Lever III <chuck.lever@oracle.com>]

Hi-

> On Jun 28, 2021, at 2:17 PM, Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Mon, Jun 28, 2021 at 02:12:59PM -0400, Chuck Lever wrote:
>> The SUNRPC consumer of __alloc_bulk_pages() legitimately calls it
>> with a full array sometimes. In that case, the correct return code,
>> according to the API contract, is to return the number of pages
>> already in the array/list.
>> 
>> Let's clean up the return logic to make it clear that the returned
>> value is always the total number of pages in the array/list, not the
>> number of pages that were allocated during this call.
> 
> This is more complicated than either v1 or the version that Mel sent
> earlier today.  Is it worth it?

Yes.

My v2 addresses the reason the bug was introduced in the first place: The
code currently does not reflect the API contract described in the documenting
comment.

A human reading the function as it is currently written might easily expect
that a zero return code is proper if something failed. However, the API
contract does not list zero as a unique return value with a special meaning.
The contract merely states: "Returns the number of pages on the list or
array." Therefore zero is a plausible return value only if @nr_pages is
zero or less.

Note that the value returned if prepare_alloc_pages() fails is also incorrect,
by my reading, and my v2 addresses that.

The only other call site is __page_pool_alloc_pages_slow(), and that looks
incorrect to me -- it does not agree with either the API contract or the
SUNRPC call site. I did not fix that, but I think it should be looked into
by someone familiar with that code.

I haven't seen the patch Mel sent earlier today. I was not cc'd on that one
or on b3b64ebd3822.

--
Chuck Lever