Re: Warning on memory offline (and possible in usual migration?)

[Nick Piggin <npiggin@suse.de>]

On Tue, Apr 22, 2008 at 04:56:08PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 22 Apr 2008 06:52:05 +0200
> Nick Piggin <npiggin@suse.de> wrote:
> > > I guess BUG_ON(!PageUptodate) would be better here?
> > > 
> > > > <snip>
> > > >         if (PageDirty(page)) {
> > > >                 clear_page_dirty_for_io(page);
> > > >                 set_page_dirty(newpage);------------------------(**)
> > > >         }
> > > > 
> > > > ==
> > > > Then, Uptodate() is copied before set_page_dirty(). 
> > > > So, "page" is not Uptodate and Dirty when it reaches (**)
> > > 
> > > The page will be marked uptodate before we reach ** so its okay in 
> > > general. If a page is not uptodate then we should not be getting here.
> > > 
> > > An !uptodate page is not migratable. Maybe we need to add better checking?
> > 
> > Why is an !uptodate page not migrateable, and where are you testing to
> > prevent that?
> > 
> 
> I'm sorry if I don't understand correctly, in usual, can I consider
> !PageUptodate() page is under I/O or some unstable state ?

No, it need not be under IO or in some unstable state. Christoph just
said that migration can't handle !uptodate pages, and I'm very
curious as to why not, and what is in place to prevent that from
happening.


> If so, migration is danger.
> And set_page_dirty() shown WARNING when I migrated a dirty and not-up-to-date
> page. To avoid WARNING, a not-up-to-date page should not be migratable.
> 
> 
> I tested PageUptodate() in
> ==
> static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>                         struct page *page, int force)
> {
>     <snip>
>         if (TestSetPageLocked(page)) {
>                 if (!force)
>                         goto move_newpage;
>                 lock_page(page);
>         }
>       (*) here
> ....
> }
> ==
> and the page never becomes Uptodate.
> 
> Thanks,
> -Kame

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