Re: page faults

[James Simmons <jsimmons@edgeglobal.com>]

On Fri, 22 Oct 1999, Stephen C. Tweedie wrote:

> Hi,
> 
> On Thu, 21 Oct 1999 15:40:15 -0400 (EDT), "Benjamin C.R. LaHaise"
> <blah@kvack.org> said:
> 
> > On Thu, 21 Oct 1999, James Simmons wrote:
> >> Quick question. If two processes are sharing the same memory but no page
> >> fault has happened. THen process A causes a page fault. If process B tries
> >> to access the page that process A already page fault will process B cause
> >> another page fault. Or do page faults only happen once no matter how many
> >> process access it. 
> 
> > Only the first time the page is accessed is there a fault to put the entry
> > into the page table, regardless of the processes sharing the page.  
> 
> No.  If a process mmap()s a file and forks, then the two processes will
> page fault independently.  Similarly if two separate processes mmap()
> the same file, they will page fault independently.  Finally, if a
> process has a data page which becomes faulted out and the process forks,
> then the two resuling processes will both have to take independent page
> faults to map the page.
> 
> However, there will only ever be one major fault (ie. one fault which
> has to bring data in from disk).  If multiple processes share the same
> page, then the second and all subsequent processes to fault on that page
> will take a minor page fault which will just find the existing page in
> memory and map that into the faulting process's page tables.

Thank you for that answer. I remember you told me that threads under linux
is defined as two processes sharing the same memory. So when a minor
page fault happens by anyone one process will both process page tables get
updated? Or does the other process will have a minor page itself
independent of the other process? 

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