mm/vmalloc: clarify why vmap_range_noflush() might sleep

The only reason vmap_range_noflush() can sleep is because of pagetable
allocations.

The actual allocation mechanism is arch-specific so might_alloc() doesn't
work here (what GFP flags would be used?).  Hence, just add a comment.

Also note that this might do a TLB shootdown.  This is not actually
sleeping but it requires IRQs on for x86, and might_sleep() incidentally
serves to detect violations of that too.

Link: https://lkml.kernel.org/r/20251215-b4-vmalloc-might_alloc-v3-1-92dd8e406868@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -305,6 +305,11 @@ static int vmap_range_noflush(unsigned long addr, unsigned long end,
 	int err;
 	pgtbl_mod_mask mask = 0;
 
+	/*
+	 * Might allocate pagetables (for most archs a more precise annotation
+	 * would be might_alloc(GFP_PGTABLE_KERNEL)). Also might shootdown TLB
+	 * (requires IRQs enabled on x86).
+	 */
 	might_sleep();
 	BUG_ON(addr >= end);