This issue is observed during a CHECHKPOINT when a BGWriter writes dirty or modified buffers from shared buffers to Disk. So, upon recovery after crash, the cha

This issue is observed during a CHECHKPOINT when a BGWriter writes dirty or modified buffers from shared buffers to Disk. So, upon recovery after crash, the changes since the last checkpoint are applied from WAL, this time. So, we are using the changes that are in a WAL (Write-ahead-log) because the changes are already gone from Disk as you rightly said. A good question in fact.