Re: filecache/swapcache questions

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> Hi,
> 
> On Mon, 21 Jun 1999 11:46:27 -0700 (PDT), kanoj@google.engr.sgi.com
> (Kanoj Sarcar) said:
> 
> >> Look no further than swap_in(), which knows that there is no pte (so
> >> swapout concurrency is not a problem) and it holds the mmap lock (so
> >> there are no concurrent swap_ins on the page).  It reads in the page adn
> >> unconditionally sets up the pte to point to it, assuming that nobody
> >> else can conceivably set the pte while we do the swap outselves.
> 
> > Hmm, am I being fooled by the comment in swap_in?
> 
> > /*
> >  * The tests may look silly, but it essentially makes sure that
> >  * no other process did a swap-in on us just as we were waiting.
> >  *
> 
> afaik only swapoff can trigger that.  Concurrent swap-in on the same
> entry can occur into the page cache, but not into the page tables
> because those are protected by the semaphore.
> 
> --Stephen
> 

Right ... I was trying to counter your argument that swapoff needs
to hold the mmap_sem to protect ptes (except for the fork/exit/swapin 
races) by pointing out that pte updates are already protected by 
kernel_lock.

Kanoj
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