Catching when the linux kernel writes a page back to a memory mapped file?

Question

I'm contemplating a system that would let me memory map files and transparently do type conversion on the data they contain. It seems it's possible to catch memory accesses by mmaping a second memory region and making it protected, then catching the segfault when a new page is accessed. This would let me handle the on-read type conversion I need.

However, to be read/write compatible, I'd need some way to catch when the OS is paging part of the memory back to disk so I could do the type conversion the other way before it's written.

Is there any capability for hooking the paging system in this way?

Answer 1

What you want is not possible, and reflects a fundamental misunderstanding of mmap. The event of file-backed maps being written back on disk is not relevant, because until this happens, any attempt to read the file will (and must, to conform to POSIX) be read from the modified in-memory copy of the page, not the outdated contents on disk. In other words, the writing back of modified pages to disk is completely transparent to applications, and assuming you never lose power or reboot, it would be completely possible that the modified page is never written back to disk.

Your design just doesn't work. You'll have to do something different if you want this kind of behavior.