vm_pageout: simplify pageout_cluster

[dougm]

Summary

Rewrite vm_pageout_cluster to eliminate redundant variables and duplicated code, without changing its functionality.

Remove tests on pindex to check for object boundary conditions, since the page_next and page_prev functions return NULL at the object boundaries. Fix an alignment error that could happen if pindex is aligned, and the first of vm_pageout_page_count flushable pages, and the page at pindex-1 is also flushable.

[alc]

For the most part, I am fine with this. Personally, I would prefer to see the NULL check remain in the caller, and I dislike the ok2cluster in vm_pageout_ok2cluster.

[dougm]

Amend to address @alc comments.

[dougm]

Add a comment to explain the ib > pindex test.

[dougm]

sys/vm/vm_pageout.c

398

Actually, this test is unnecessary. If ib > pindex, then m->pindex == 0 and vm_page_prev(m) is NULL, so the break will happen anyway.

[dougm]

sys/vm/vm_pageout.c

421

The object->size test is unnecessary, since if pindex + is == object->size, vm_page_next(m) will be NULL and terminate the loop.

Better to do that, and set a right_terminated flag before the break.

Then, in the first loop, move the % vm_pageout_page_count test to the top of the loop, but only test it if right_terminated is false.

And replace "ib == 0" with left_terminated ==true, and get rid of ib and is variables.

[dougm]

More changes, to remove needless tests and more variables.

[dougm]

Rewrite the alignment-test expression to make it more readable.

[dougm]

Move the "% vm_pageout_page_count" calculation out of the loop.

[dougm]

Remove a redundant 'num_ends' check from the alignment test.

[dougm]

Undo the last change, since it breaks the case with the first iteration of the forward loop fails.