On Thu, 25 Jul 2013 16:57:44 -0700, kernel neophyte said: > I am sorry, its still not clear to me. All I am asking is I want to know > and understand how mmap works, given an address *X*, how does the Linux > kernel figure out that *X* is an mmaped page? Before you ask *how* it does it, first figure out if it does it *at all*. The reason you can't figure out how the swami is levitating is because they aren't actually levitating at all. > Is there a special flag in > the page table entry? No, because no flag is needed. > Does the access generate a page fault ? Maybe, maybe not. If the page is resident in memory there's no page fault. And if it's not resident, it gets paged in from wherever the backing page happens to be. > If so, how > does the handler find out it is an mmaped address? The handler doesn't *care* if it's mmaped. All it has to know is (1) this page isn't in memory, (2) it needs to be in memory, and (3) so please schedule the I/O to read it from block NNNN of device XXYY just like any *other* page being read in because of a page fault. The *only* thing "magical" about an mmap'ed page is that the pointer to where to read/write it might (sometimes) point at someplace that's not a swap space. Though for some uses of mmap(), it *does* point at swap space (for instance, the anonymous pages created by mmap() as used from malloc() in glibc).
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