Meaning of the dirty bit

[Martin Maletinsky <maletinsky@scs.ch>]

Hi,

While studying the follow_page() function (the version of the function that is in place since 2.4.4, i.e. with the write argument), I noticed, that for an address that
should be written to (i.e. write != 0), the function checks not only the writeable flag (with pte_write()), but also the dirty flag (with pte_dirty()) of the page
containing this address.
From what I thought to understand from general paging theory, the dirty flag of a page is set, when its content in physical memory differs from its backing on the permanent
storage system (file or swap space). Based on this understanding I do not understand why it is necessary to check the dirty flag, in order to ensure that a page is writable
- what am I missing here?

Thanks in advance for any answers
with best regards
Martin Maletinsky

P.S. Pls. put me on cc: in your reply, since I am not on the mailing list.

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