mmap implementation for writing

The mmap(2) man page seems to say that the file fields showing that the file was modified will be updated after a write to the mmap'd area but before any subsequent msync().

How is this done?

The only way I can see so that linux knows that a memory write to the mmap'd area has occurred is to turn off write permission on the page, thus causing a fault on write access. At which point linux realizes that the write was legitimate, updates the file's field flags, resets the permissions on the page, lets the write go through and then, somehow, takes the write permissions back off the page.

Have I misunderstood the man page?

Roy

Please be so kind as not to tell me to read the kernel code. I don't know the code and the ramp-up effort would be pretty extensive for just simply wanting to know in outline how it's done (if it is). Thanks.

----------------------- mmap(2) ---------------------------

The stctime and stmtime field for a file mapped with PROTWRITE and MAPSHARED will be updated after a write to the mapped region, and before a subsequent msync() with the MSSYNC or MSASYNC flag, if one occurs.

-------------------------------------------------------------