mm/vmalloc: add more comments to the adjust_va_to_fit_type()

When fit type is NE_FIT_TYPE there is a need in one extra object.
Usually the "ne_fit_preload_node" per-CPU variable has it and there is
no need in GFP_NOWAIT allocation, but there are exceptions.

This commit just adds more explanations, as a result giving answers on
questions like when it can occur, how often, under which conditions and
what happens if GFP_NOWAIT gets failed.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Uladzislau Rezki (Sony) <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Daniel Wagner <[e-mail]>
Cc: Sebastian Andrzej Siewior <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Uladzislau Rezki <[e-mail]>
Cc: Hillf Danton <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Oleksiy Avramchenko <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmalloc.c

···
 			 * There are a few exceptions though, as an example it is
 			 * a first allocation (early boot up) when we have "one"
 			 * big free space that has to be split.
+			 *
+			 * Also we can hit this path in case of regular "vmap"
+			 * allocations, if "this" current CPU was not preloaded.
+			 * See the comment in alloc_vmap_area() why. If so, then
+			 * GFP_NOWAIT is used instead to get an extra object for
+			 * split purpose. That is rare and most time does not
+			 * occur.
+			 *
+			 * What happens if an allocation gets failed. Basically,
+			 * an "overflow" path is triggered to purge lazily freed
+			 * areas to free some memory, then, the "retry" path is
+			 * triggered to repeat one more time. See more details
+			 * in alloc_vmap_area() function.
 			 */
 			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
 			if (!lva)