Re: page_launder() bug

[Marcelo Tosatti]

On Sun, 13 May 2001, Linus Torvalds wrote:

> 
> On Sun, 13 May 2001, Rik van Riel wrote:
> > 
> > This means that the swapin path (and the same path for
> > other pagecache pages) doesn't take the page lock and
> > the page lock doesn't protect us from other people using
> > the page while we have it locked.
> 
> You can test for swap cache deadness without holding the page cache lock:
> if the swap count is 1, then we know that nobody else has this swap entry
> in its page tables, and thus there can not be any concurrent lookups
> either.
> 
> Now, it may well be that we need to make sure that there is some proper
> ordering (nobody must decrement the swap count before they increment the
> page count or something). I think that is the case anyway (and I _think_
> that everybody that mucks with the swap count always hold the page count -
> this might be a good thing to check).

Swapin readahead _first_ increases the swap map count for the given page
and then increases the page count.