[PATCH 1/2] mm/vmalloc: free unused pages when shrinking vrealloc() allocation

[Jill Ravaliya <jillravaliya@gmail.com>]

vrealloc() shrink path zeros unused memory and updates
vm->requested_size, but never frees the physical pages,
removes page table mappings, or flushes the TLB for the
unused range.

When a caller shrinks a vmalloc allocation, physical pages
backing the unused portion remain allocated until vfree()
is eventually called, wasting real RAM.

Fix this by unmapping the unused virtual range using
vunmap_range() which also flushes the TLB, freeing each
unused physical page back to the buddy allocator, and
updating vm->nr_pages to reflect the new page count.

Signed-off-by: Jill Ravaliya <jillravaliya@gmail.com>
---
 mm/vmalloc.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index aa08651ec..a8cedfc5d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4336,6 +4336,27 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 			memset((void *)p + size, 0, old_size - size);
 		vm->requested_size = size;
 		kasan_vrealloc(p, old_size, size);
+
+		/* Shrink the vm_area: unmap and free unused pages. */
+		if (size < alloced_size) {
+			unsigned long new_nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
+			unsigned long i;
+
+			/* Unmap unused virtual range and flush TLB. */
+			vunmap_range((unsigned long)p + PAGE_ALIGN(size),
+				     (unsigned long)p + alloced_size);
+
+			/* Free unused physical pages back to buddy allocator. */
+			for (i = new_nr_pages; i < vm->nr_pages; i++) {
+				mod_lruvec_page_state(vm->pages[i],
+						      NR_VMALLOC, -1);
+				__free_page(vm->pages[i]);
+				vm->pages[i] = NULL;
+			}
+
+			vm->nr_pages = new_nr_pages;
+		}
+
 		return (void *)p;
 	}
 
-- 
2.43.0