Re: dm-bufio: Allow clients to specify an upper bound on cache size.

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Sep 06 2019 at  3:45am -0400,
Martijn Coenen <maco@android.com> wrote:

> The upper limit on the cache size of a client is currently determined by
> dividing the total cache size by the number of clients. However, in some
> cases it is beneficial to give one client a higher limit than others; an
> example is a device with many dm-verity targets, where one target has a
> very large hashtree, and all the others have a small hashtree. Giving
> the target with the large hashtree a higher limit will be beneficial.
> Another example is dm-verity-fec: FEC is only used in (rare) error
> conditions, yet for every dm-verity target with FEC, we create two FEC
> dm-bufio clients, which together have a higher cache limit than the
> dm-verity target itself.
> 
> This patchset allows a client to indicate a maximum cache size for its
> client; if that maximum is lower than the calculated per-client limit,
> that maximum will be used instead, and the freed up cache size will be
> allocated to other clients (that haven't set a maximum).
> 
> Note that this algorithm is not perfect; if we have 100MB with 3
> clients, where the first set a max of 1MB, the second set a max of 40MB,
> and the third set no maximumm, the ideal allocation would be 1:40:59,
> respectively. However, because the initial per-client limit is 100 / 3
> =~33MB, the requested max of 40MB is over the per-client limit, and
> instead the allocation will end up being ~ 1:40:49. This is still better
> than the original 33:33:33 allocation. An iterative algorithm could do
> better, but it also complicates the code significantly.

Definitely not very intuitive.. but yes I think it is a reasonable
tradeoff between your goals and further code complexity to be able to
achieve the "ideal".

Think the documented example can be made clearer by documenting that
dm_bufio_cache_size_per_client = 49.  And that _that_ is the reason why
the client that didn't set a maximum is bounded to 49.

Overall I think this patch looks reasonable, but I'd like Mikulas to
review this closer before I pick it up.

Thanks,
Mike