What does it mean that a page fault may or may not involve auxillary storage access in making that page available

Question

From manpage of top

nMaj  --  Major Page Fault Count
           The number of major page faults that have occurred for a task.  A
           page fault occurs when a process attempts to read from or write
           to a virtual page that is not currently present in its address
           space.  **A major page fault is when auxiliary storage access is
           involved in making that page available.**

nMin  --  Minor Page Fault count
           The number of minor page faults that have occurred for a task.  A
           page fault occurs when a process attempts to read from or write
           to a virtual page that is not currently present in its address
           space.  **A minor page fault does not involve auxiliary storage
           access in making that page available.**

If I am correct, handling page fault is to transfer needed data missing in the physical memory from the swap to the physical memory. Swap is part of the storage e.g. hard drive or SSD. So what does it mean that a page fault may or may not involve auxillary storage access in making that page available?

Thanks.

Answer 1

Only major page faults involve auxiliary storage (i.e. reading from disk, either from swap or elsewhere, e.g. when paging in a binary).

Minor page faults are faults which can be satisfied without reading from disk:

• page faults involving data which is already mapped elsewhere in memory (e.g. memory which can be shared between processes)
• page faults for newly-allocated memory (using on an all-zero page and copy-on-write allocation)
• in some cases, data which has been marked for paging out but hasn't yet been paged out

(this isn't exhaustive).

Answer 2

A minor page fault happens for instance, when the data or code is in memory, but not mapped to the process.

Think of a share library, utilised by another process. Your process wants to load the same lib, so the ld (i think via libc/glibc?) issues generates a page fault. The kernel knows the data is already in mem, and maps it to the process. No need to load it from disk...