Re: Oops in __free_pages_ok (pre7-1) (Long) (backtrace)

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

> 
> 
> 
> On Wed, 3 May 2000, Kanoj Sarcar wrote:
> > 
> > Note that try_to_swap_out holds the vmlist/page_table_lock on the
> > victim process, as well as lock_kernel, and though this is not the
> > easiest code to analyze, it seems to me that is enough protection 
> > on the swapcache pages.
> 
> The swapcache code gets none of those locks as far as I can tell.
> 
> The swapcache code gets the page lock, and the "page cache" lock. But it
> doesn't get the vmlist lock (the swap cache is not associated withany
> particular mm), nor does it get the kernel lock (I think - I didn't look
> through the code-paths).
>

What we are coming down to is a case by case analysis. For example,
do_wp_page, which does pull a page out of the swap cache, has the
vmlist_lock. do_swap_page does not, but neither is the page in the
pte at that point. free_page_and_swap_cache already has the vmlist_lock.

Some of this is documented in Documentation/vm/locking under the
section "Swap cache locking".

Kanoj
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