/proc/pid/pagemap. This file lets a userspace process find out which physical frame each virtual page is mapped to. It contains one 64-bit value for each virtual page, containing the following data (from fs/proc/task_mmu.c, above pagemap_read):

Bits 0-54 page frame number (PFN) if present

Bits 0-4 swap type if swapped

Bits 5-54 swap offset if swapped

Bit 55 pte is soft-dirty (see Documentation/admin-guide/mm/soft-dirty.rst)

Bit 56 page exclusively mapped (since 4.2)

Bit 57 pte is uffd-wp write-protected (since 5.13) (see Documentation/admin-guide/mm/userfaultfd.rst)

Bits 58-60 zero

Bit 61 page is file-page or shared-anon (since 3.5)

Bit 62 page swapped

Bit 63 page present

Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs. In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from 4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN. Reason: information about PFNs helps in exploiting Rowhammer vulnerability.

If the page is not present but in swap, then the PFN contains an encoding of the swap file number and the page’s offset into the swap. Unmapped pages return a null PFN. This allows determining precisely which pages are mapped (or in swap) and comparing mapped pages between processes.

Efficient users of this interface will use /proc/pid/maps to determine which areas of memory are actually mapped and llseek to skip over unmapped regions.

/proc/kpagecount. This file contains a 64-bit count of the number of times each page is mapped, indexed by PFN.