dirty page
also: modified page, unwritten page
A memory page that has been modified by a process but not yet written back to disk. The kernel tracks dirty pages and flushes them periodically or on demand.
A dirty page is a portion of RAM (typically 4 KB) that contains data that differs from what's stored on disk. When a process writes to a file, the kernel initially stores changes in memory for performance reasons rather than writing immediately to storage.
The kernel maintains a dirty page list and periodically syncs these pages to disk through background processes like kthreadd or pdflush (or kworker in modern kernels). You can monitor dirty pages using /proc/meminfo or tools like vmstat.
Example: If you edit a file and write 100 KB of data, those pages become dirty in memory. The kernel waits a few seconds before flushing them to disk, improving performance by batching writes. A system crash before flushing means those changes are lost.