Re: [PATCH v6 01/19] mm: Return void from various readahead functions

John Hubbard
2020-02-18T21:52:54Z

On 2/18/20 1:21 PM, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 01:05:29PM -0800, John Hubbard wrote:
>> This is an easy review and obviously correct, so:
>>
>> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
>
> Thanks
>
>> Thoughts for the future of the API:
>>
>> I will add that I could envision another patchset that went in the
>> opposite direction, and attempted to preserve the information about
>> how many pages were successfully read ahead. And that would be nice
>> to have (at least IMHO), even all the way out to the syscall level,
>> especially for the readahead syscall.
>
> Right, and that was where I went initially. It turns out to be a
> non-trivial aount of work to do the book-keeping to find out how many
> pages were _attempted_, and since we don't wait for the I/O to complete,
> we don't know how many _succeeded_, and we also don't know how many
> weren't attempted because they were already there, and how many weren't
> attempted because somebody else has raced with us and is going to attempt
> them themselves, and how many weren't attempted because we just ran out
> of memory, and decided to give up.
>
> Also, we don't know how many pages were successfully read, and then the
> system decided to evict before the program found out how many were read,
> let alone before it did any action based on that.
>

That is even worse than I initially thought. :)

> So, given all that complexity, and the fact that nobody actually does
> anything with the limited and incorrect information we tried to provide
> today, I think it's fair to say that anybody who wants to start to do
> anything with that information can delve into all the complexity around
> "what number should we return, and what does it really mean". In the

Yes, and now that you mention it, it's really tough to pick a single number
that answers the right questions that the user space caller might have. whew.

> meantime, let's just ditch the complexity and pretense that this number
> means anything.
>

Definitely. Thanks for the notes here.

thanks,
--
John Hubbard
NVIDIA