Re: filecache/swapcache questions

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

> 
> Hi,
> 
> On Mon, 21 Jun 1999 09:46:19 -0700 (PDT), kanoj@google.engr.sgi.com
> (Kanoj Sarcar) said:
> 
> > I don't agree with you about swapoff needing the mmap_sem. In my
> > thinking, mmap_sem is needed to preserve the vma list, *if* you 
> > go to sleep while scanning the list. Updates to the vma fields/
> > chain are protected by kernel_lock and mmap_sem. 
> 
> No.  mmap_sem protects both the vma list and the page tables.  Page
> faults hold the mmap semaphore both to protect the vma list and to
> protect against concurrent pagins to the same page.  
> 
> The swapper is currently exempt from the mmap_sem, so the paging code
> needs to check whether the current pte has disappeared if it ever
> blocks, but it assumes that we never have concurrent pagein occurring
> (think threads).  swapoff currently breaks that assumption.
>

But doesn't my previous logic work in this case too? Namely
that kernel_lock is held when any code looks at or changes
a pte, so if swapoff holds the kernel_lock and never goes to 
sleep, things should work?

Maybe if you can jot down a quick scenario where a problem occurs
when swapoff does not take mmap_sem, it would be easier for me
to spot which concurrency issue I am missing ...

Thanks.

Kanoj
kanoj@engr.sgi.com
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