Re: [PATCH v2 00/11] mempolicy2, mbind2, and weighted interleave

Gregory Price
2023-12-12T15:59:21Z

On Tue, Dec 12, 2023 at 03:08:24PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@memverge.com> writes:
>
> >> For example, can we use something as below?
> >>
> >> long set_mempolicy2(int mode, const unsigned long *nodemask, unsigned int *il_weights,
> >> unsigned long maxnode, unsigned long home_node,
> >> unsigned long flags);
> >>
> >> long mbind2(unsigned long start, unsigned long len,
> >> int mode, const unsigned long *nodemask, unsigned int *il_weights,
> >> unsigned long maxnode, unsigned long home_node,
> >> unsigned long flags);
> >>
> >
> > Your definition of mbind2 is impossible.
> >
> > Neither of these interfaces solve the extensibility issue. If a new
> > policy which requires a new format of data arrives, we can look forward
> > to set_mempolicy3 and mbind3.
>
> IIUC, we will not over-engineering too much. It's hard to predict the
> requirements in the future.
>

Sure, but having the mempolicy struct at least gives us more flexibility
than the original interface.

> >> A struct may be defined to hold mempolicy iteself.
> >>
> >> struct mpol {
> >> int mode;
> >> unsigned int home_node;
> >> const unsigned long *nodemask;
> >> unsigned int *il_weights;
> >> unsigned int maxnode;
> >> };
> >>
> >
> > addr could be pulled out for get_mempolicy2, so i will do that
> >
> > 'addr_node' and 'policy_node' are warts that came from the original
> > get_mempolicy. Removing them increases the complexity of handling
> > arguments in the common get_mempolicy code.
> >
> > I could probably just drop support for retrieving the addr_node from
> > get_mempolicy2, since it's already possible with get_mempolicy. So I
> > will do that.
>
> If it's necessary, we can add another struct for get_mempolicy2(). But
> I don't think that it's necessary to add get_mempolicy2() specific
> parameters for set_mempolicy2() or mbind2().

After edits, the only parameter that doesn't have parity between
interfaces is `addr_node` and `policy_node`. This was an unfortunate
wart on the original get_mempolicy() that multiplexed the output of
(*mode) based on whether MPOL_F_NODE was set.

Example:
if (MPOL_F_ADDR | MPOL_F_NODE), then get_mempolicy() would return
details about a VMA mempolicy + the node of that address in (*mode).

Right now in get_mempolicy2() I fetch this unconditionally instead of
requiring MPOL_F_NODE. I did not want to multiplexing (*mode) output.

I see two options:
1) Get rid of MPOL_F_NODE functionality in get_mempolicy2()
If a user wants that information, they can still use get_mempolicy()

2) Keep MPOL_F_NODE and mpol_args->addr_node/policy_node, but don't allow
any future extensions that create this kind of situation.

I'm fine with either. I originally aimed for get_mempolicy2() to be
all of get_mempolicy() features + new data, but that obviously isn't
required.

~Gregory