mm/vmalloc: fix page mapping if vm_area_alloc_pages() with high order fallback to order 0

The __vmap_pages_range_noflush() assumes its argument pages** contains
pages with the same page shift.  However, since commit e9c3cda ("mm,
vmalloc: fix high order __GFP_NOFAIL allocations"), if gfp_flags includes
__GFP_NOFAIL with high order in vm_area_alloc_pages() and page allocation
failed for high order, the pages** may contain two different page shifts
(high order and order-0).  This could lead __vmap_pages_range_noflush() to
perform incorrect mappings, potentially resulting in memory corruption.

Users might encounter this as follows (vmap_allow_huge = true, 2M is for
PMD_SIZE):

kvmalloc(2M, __GFP_NOFAIL|GFP_X)
    __vmalloc_node_range_noprof(vm_flags=VM_ALLOW_HUGE_VMAP)
        vm_area_alloc_pages(order=9) ---> order-9 allocation failed and fallback to order-0
            vmap_pages_range()
                vmap_pages_range_noflush()
                    __vmap_pages_range_noflush(page_shift = 21) ----> wrong mapping happens

We can remove the fallback code because if a high-order allocation fails,
__vmalloc_node_range_noprof() will retry with order-0.  Therefore, it is
unnecessary to fallback to order-0 here.  Therefore, fix this by removing
the fallback code.

Link: https://lkml.kernel.org/r/20240808122019.3361-1-hailong.liu@oppo.com
Fixes: e9c3cda ("mm, vmalloc: fix high order __GFP_NOFAIL allocations")
Signed-off-by: Hailong Liu <hailong.liu@oppo.com>
Reported-by: Tangquan Zheng <zhengtangquan@oppo.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Acked-by: Barry Song <baohua@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: hailong-liu

mm/vmalloc.c

@@ -3584,15 +3584,8 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 			page = alloc_pages_noprof(alloc_gfp, order);
 		else
 			page = alloc_pages_node_noprof(nid, alloc_gfp, order);
-		if (unlikely(!page)) {
-			if (!nofail)
-				break;
-
-			/* fall back to the zero order allocations */
-			alloc_gfp |= __GFP_NOFAIL;
-			order = 0;
-			continue;
-		}
+		if (unlikely(!page))
+			break;
 
 		/*
 		 * Higher order allocations must be able to be treated as