kernel/vm: when resizing down a VMCache, fill a partial page with zeros

* factored out file_cache
* fix two gVisor tests: MMapFileTest.ReadSharedTruncatePartialPage
and MMapFileTest.WriteSharedTruncatePartialPage
* fix #19438

Change-Id: Ia7f119478d306ab6d867632e4a5270733f0b29df
Reviewed-on: https://review.haiku-os.org/c/haiku/+/9060
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/cache/file_cache.cpp

@@ -1276,23 +1276,9 @@ file_cache_set_size(void* _cacheRef, off_t newSize)
 	VMCache* cache = ref->cache;
 	AutoLocker<VMCache> _(cache);
 
-	off_t oldSize = cache->virtual_end;
 	status_t status = cache->Resize(newSize, VM_PRIORITY_USER);
 		// Note, the priority doesn't really matter, since this cache doesn't
 		// reserve any memory.
-	if (status == B_OK && newSize < oldSize) {
-		// We may have a new partial page at the end of the cache that must be
-		// cleared.
-		uint32 partialBytes = newSize % B_PAGE_SIZE;
-		if (partialBytes != 0) {
-			vm_page* page = cache->LookupPage(newSize - partialBytes);
-			if (page != NULL) {
-				vm_memset_physical(page->physical_page_number * B_PAGE_SIZE
-					+ partialBytes, 0, B_PAGE_SIZE - partialBytes);
-			}
-		}
-	}
-
 	return status;
 }
 

src/system/kernel/vm/VMCache.cpp

@@ -1172,6 +1172,17 @@ VMCache::Resize(off_t newSize, int priority)
 		while (_FreePageRange(pages.GetIterator(newPageCount, true, true)))
 			;
 	}
+	if (newSize < virtual_end && newPageCount > 0) {
+		// We may have a partial page at the end of the cache that must be cleared.
+		uint32 partialBytes = newSize % B_PAGE_SIZE;
+		if (partialBytes != 0) {
+			vm_page* page = LookupPage(newSize - partialBytes);
+			if (page != NULL) {
+				vm_memset_physical(page->physical_page_number * B_PAGE_SIZE
+					+ partialBytes, 0, B_PAGE_SIZE - partialBytes);
+			}
+		}
+	}
 
 	if (priority >= 0) {
 		status_t status = Commit(PAGE_ALIGN(newSize - virtual_base), priority);