mmap how does kernel know a page is mmaped

[Valdis.Kletnieks at vt.edu]

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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