Memory page deallocation in case of faults refers to how an operating system handles the removal and cleanup of memory pages when something goes wrong—typically during a page fault or when a page is deemed invalid, corrupted, or no longer needed.

Let’s break it down clearly.

1. What is a page fault?

A page fault occurs when a program tries to access a memory page that is:

• Not currently in physical RAM (valid but swapped out), or
• Not mapped at all (invalid access)

The CPU traps into the OS to handle it.

2. What does “page deallocation” mean here?

Page deallocation means the OS:

• Frees the physical memory frame
• Removes the page table entry mapping
• Marks the page as invalid/unusable
• Returns the frame to the free memory pool

It is essentially:

“This page is no longer needed or is invalid, so reclaim its memory.”

3. When does deallocation happen during faults?

(A) Invalid page fault (segmentation-style fault)

If the fault is caused by accessing an illegal address:

• OS detects the page is not part of process address space
• Process is usually terminated (or signal sent)
• Any partially allocated page structures may be cleaned up
• Memory is reclaimed immediately

👉 Example: accessing freed pointer

(B) Fault during memory protection violation

If access violates permissions (e.g., write to read-only page):

• OS does NOT normally deallocate the page
• Instead, it may terminate or adjust permissions (rare cases like copy-on-write)

So deallocation is not always involved.

(C) Page replacement under memory pressure (related, not fault-triggered)

When RAM is full:

• OS may evict a page (write to disk if dirty)
• Then deallocate the frame (free it for reuse)

This is more “page replacement” than fault handling, but closely related.

4. What exactly is freed?

When a page is deallocated, OS updates:

• Page table entry → invalid
• Frame table → marked free
• Cache/TLB entries → invalidated
• Swap metadata (if used) → cleared or updated

5. Simple mental model

Think of RAM like hotel rooms:

• Each page = a room occupant
• Page fault = someone tries to enter a room
• If invalid guest → they are removed and room is freed (deallocation)
• If valid guest but room not in hotel → OS brings them in
• If hotel is full → some guests are evicted and rooms freed

6. Key takeaway

Memory page deallocation during faults is the OS process of reclaiming physical memory frames when a page access is invalid or when recovery/cleanup is required after a fault.