From my understanding, when Linux swaps a physical page frame in/out RAM, it needs to set valid bit for all virtual pages mapping into this physical page. Mapping a virtual page to physical page frame seems to be well explained in textbooks, but how does the kernel find all virtual pages from a physical page frame? Actual implementation in Linux source code would be appreciated.

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Each physical page of memory is tracked in the kernel using struct page. This allows the kernel to describe how each page is used; in particular, for anonymous and file-backed mappings, the mapping field points to the address_space structure used to describe the mapped object.

For code which needs to find virtual mappings using a given physical page, the kernel provides a set of reverse mapping functions. These allow the reverse mappings for anonymous mappings and file-backed mappings to be walked. For example, try_to_unmap walks the maps looking for any use of a given physical page, so that it can unmap it. shrink_page_list calls try_to_unmap when it decides it needs to unmap a page which is mapped into processes.

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