Re: Meaning of the dirty bit

[Martin Maletinsky <maletinsky@scs.ch>]

Hello,

Thanks for your reply. What is the reason to check the dirty bit in follow_page(), which (presumably) should just parse the page tables, verify write access (if the write
argument is set) and return the page descriptor describing the page the address is in (from what I understood, there is no I/O involved).
Is there any reason to deny write access when the dirty flag is not set?

Thanks again,
regards
Martin

Dharmender Rai wrote:

> Hi,
> The purpose is to achieve need-based disk I/O.
> Dirty-flag-set means you have to write the contents of
> that page to the disk before paging out or
> invalidating that page. If the dirty flag is not set
> then there is no need for the I/O part.
>
> Regards
> Dharmender Rai
>
>  --- Martin Maletinsky <maletinsky@scs.ch> wrote: >
> 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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> > Martin Maletinsky                 phone: +41 (0)1
> > 445 16 05
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> > 445 16 10
> > CH-8005 Zurich
> >
> >
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--
Supercomputing System AG          email: maletinsky@scs.ch
Martin Maletinsky                 phone: +41 (0)1 445 16 05
Technoparkstrasse 1               fax:   +41 (0)1 445 16 10
CH-8005 Zurich


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