Messages in this thread | | | Subject | Re: [PATCH] take pageout refcount into account for remove_exclusive_swap_page | From | Lee Schermerhorn <> | Date | Thu, 15 May 2008 13:55:24 -0400 |
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On Thu, 2008-05-15 at 11:15 +0900, Daisuke Nishimura wrote: > On 2008/05/14 4:02 +0900, Lee Schermerhorn wrote: > > On Tue, 2008-05-13 at 14:09 -0400, Rik van Riel wrote: > >> On Tue, 13 May 2008 13:43:55 -0400 > >> Lee Schermerhorn <Lee.Schermerhorn@hp.com> wrote: > >> > >>> Or, more generally, 2 + N, for an anon page that is mapped [must be > >>> read-only, right?] by N processes. This can happen after, e.g., fork(). > >>> Looks like this patch handles the condition just fine, but you might > >>> want to reflect this in the comment. > >> No, this patch only removes a page from the swap cache that is mapped > >> by one process. The function page_mapped() returns either 1 or 0, not > >> the same as page_mapcount(). > > > > Duh! I was reading "page_mapcount()", 'cause that's what I've been > > considering using for this purpose. > > > >> > >> I am not sure if trying to handle swap cache pages that are mapped by > >> multiple processes could get us into other corner cases and think that > >> we should probably try to stick to the safe thing for now. > > I think it would be better to add a comment of > remove_exclusive_swap_page_ref() that it doesn't handle > swap caches that are mapped by multiple processes for safty. > > > > > OK. I can test the more general case down the road. > > > >> Besides, shouldn't anonymous shared pages be COW and relatively rare? > > > > Well, all anon pages are shared right after fork(), right? They only > > become private once they've been written to. I don't have a feel for > > the relative numbers of shared anon vs COWed anon--either in general or > > in the swap cache. > > > >>> Now, I think I can use this to try to remove anon pages from the swap > >>> cache when they're mlocked. > > > > I suppose I can just go ahead and use this version with my stress load > > and count the times when I could have freed the swap cache entry, but > > didn't because it's mapped in multiple address spaces. Later ... :) > > > > Lee, Rik's version looks good to me except mlocked case, but > do you have any plan to handle these cases?
Well, it's really just an optimization--freeing swap cache entry, if possible, when mlocking a page. I'll patch Rik's version to use page_mapcount() and test heavily before proposing to do this. I suppose we can also discuss whether we want a separate version that frees swap mapped by multiple tasks for the mlock case, and keep Rik's version for vmscan. However, I think that if it's safe in the mlock case, it should be safe for vmscan. We'll see.
Lee
> > > >>>> + */ > >>>> +int remove_exclusive_swap_page_ref(struct page *page) > >>>> +{ > >>>> + return remove_exclusive_swap_page_count(page, 2 + page_mapped(page)); > >>>> +} > >>>> + > >>>> +/* > >>>> * Free the swap entry like above, but also try to > >>>> * free the page cache entry if it is the last user. > >>>> */ > >>>> > >>>> All Rights Reversed > >> > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > >
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