mm/vmalloc: prevent stale TLBs in fully utilized blocks

Patch series "mm/vmalloc: Assorted fixes and improvements", v2.

this series addresses the following issues:

  1) Prevent the stale TLB problem related to fully utilized vmap blocks

  2) Avoid the double per CPU list walk in _vm_unmap_aliases()

  3) Avoid flushing dirty space over and over

  4) Add a lockless quickcheck in vb_alloc() and add missing
     READ/WRITE_ONCE() annotations

  5) Prevent overeager purging of usable vmap_blocks if
     not under memory/address space pressure.


This patch (of 6):

_vm_unmap_aliases() is used to ensure that no unflushed TLB entries for a
page are left in the system. This is required due to the lazy TLB flush
mechanism in vmalloc.

This is tried to achieve by walking the per CPU free lists, but those do
not contain fully utilized vmap blocks because they are removed from the
free list once the blocks free space became zero.

When the block is not fully unmapped then it is not on the purge list
either.

So neither the per CPU list iteration nor the purge list walk find the
block and if the page was mapped via such a block and the TLB has not yet
been flushed, the guarantee of _vm_unmap_aliases() that there are no stale
TLBs after returning is broken:

x = vb_alloc() // Removes vmap_block from free list because vb->free became 0
vb_free(x)     // Unmaps page and marks in dirty_min/max range
	       // Block has still mappings and is not put on purge list

// Page is reused
vm_unmap_aliases() // Can't find vmap block with the dirty space -> FAIL

So instead of walking the per CPU free lists, walk the per CPU xarrays
which hold pointers to _all_ active blocks in the system including those
removed from the free lists.

Link: https://lkml.kernel.org/r/20230525122342.109672430@linutronix.de
Link: https://lkml.kernel.org/r/20230525124504.573987880@linutronix.de
Fixes: db64fe02 ("mm: rewrite vmap layer")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2236,9 +2236,10 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 	for_each_possible_cpu(cpu) {
 		struct vmap_block_queue *vbq = &per_cpu(vmap_block_queue, cpu);
 		struct vmap_block *vb;
+		unsigned long idx;
 
 		rcu_read_lock();
-		list_for_each_entry_rcu(vb, &vbq->free, free_list) {
+		xa_for_each(&vbq->vmap_blocks, idx, vb) {
 			spin_lock(&vb->lock);
 			if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
 				unsigned long va_start = vb->va->va_start;