Re: page faults

["Stephen C. Tweedie" <sct@redhat.com>]

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.

--Stephen
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