Have vm_page_grab_pages() support VM_ALLOC_NOWAIT

[markj]

Summary

The return value now gives the number of pages (starting at the
requested pindex) returned. This lets sendfile() use vm_page_grab_pages(),
which is marginally more cache-friendly.

Test Plan

I tried this patch with a simple benchmark that uses an AF_UNIX
stream socket pair to simulate a sendfile() to /dev/null. Throughput
goes up by about 1.5% when sending a large (8GB) memory-resident
file.

Edit: I had previous claimed a larger improvement, but had accidentally
been testing with the patch from D11943 applied as well.

[alc]

sys/vm/vm_page.c

3181 ↗

(On Diff #31839)

I would suggest the phrase "partial prefix". I think it conveys more information.

[alc]

Brett, I added you to this change, because it will decrease the time spent in vm_radix_lookup() by your shm_open()/sendfile() test case.

[markj]

• Clarify the vm_page_grab_pages() comment.