malloc: add a routine to find out usable size without allocating

[mjg]

Summary

ZFS creates several zones, some of which unnecessarily duplicate zones provided by malloc.

The suggested routine can be used to avoid the problem. Sample user would be abd_chunk cache which defaults to 4096 but can be tuned in loader.conf

[markj]

sys/kern/kern_malloc.c

1037 ↗

(On Diff #79195)

To me the name suggests that the return type should be uma_zone_t.

1042 ↗

(On Diff #79195)

The return type is (unsigned) size_t.

[mjg]

sys/kern/kern_malloc.c

1037 ↗

(On Diff #79195)

I don't care for the name, just suggest another one and I'll use it.

1042 ↗

(On Diff #79195)

Callers can check for -1 regardless. Alternatively, I can return errnos here and the actual size in a different argument, like so: error = func(size, &malloc_size);

[markj]

sys/kern/kern_malloc.c

1034 ↗

(On Diff #79195)

"returns the number of bytes allocated for a request of the specified size"

1037 ↗

(On Diff #79195)

malloc_size() is my suggestion.

1042 ↗

(On Diff #79195)

Code analysis tools will raise warnings. Why not just return 0?

[markj]

sys/kern/kern_malloc.c

1044 ↗

(On Diff #79219)

No need for braces, or at least make it consistent with the if statement immediately following.