A folio is a Linux kernel memory-management object that represents one or more physically contiguous pages as a single unit. The kernel can manage file data in the page cache through folios, reducing the bookkeeping needed when working with groups of pages.
Folios matter because efficient page-cache management can affect file-read performance, especially when many threads access cached data at once. In kernel code, a read path may inspect or copy data from a folio while using synchronization methods such as reference counting or RCU to ensure the memory remains safe to access. Folios are an internal kernel abstraction, not a separate file format or a guarantee of faster performance on their own.