Re: [PATCH 7/8] mm: reinstate ZERO_PAGE

KAMEZAWA Hiroyuki
2009-09-09T01:46:21Z

On Tue, 8 Sep 2009 12:56:57 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> On Tue, 8 Sep 2009, KAMEZAWA Hiroyuki wrote:
> >
> > A nitpick but this was a concern you shown, IIUC.
> >
> > == __get_user_pages()..
> >
> > if (pages) {
> > pages[i] = page;
> >
> > flush_anon_page(vma, page, start);
> > flush_dcache_page(page);
> > }
> > ==
> >
> > This part will call flush_dcache_page() even when ZERO_PAGE is found.
> >
> > Don't we need to mask this ?
>
> No, it's okay to flush_dcache_page() on ZERO_PAGE: we always used to
> do that there, and the arches I remember offhand won't do anything
> with it anyway, once they see page->mapping NULL.
>
> What you're remembering, that I did object to, was the way your
> FOLL_NOZERO ended up doing
> pages[i] = NULL;
> flush_anon_page(vma, NULL, start);
> flush_dcache_page(NULL);
>
> which would cause an oops when those arches look at page->mapping.
>
> I should take another look at your FOLL_NOZERO: I may have dismissed
> it too quickly, after seeing that bug, and oopsing on x86 when
> mlocking a readonly anonymous area.
>
> Though I like that we don't _need_ to change mlock.c for reinstated
> ZERO_PAGE, this morning I'm having trouble persuading myself that
> mlocking a readonly anonymous area is too silly to optimize for.
>
> Maybe the very people who persuaded you to bring back the anonymous
> use of ZERO_PAGE, are also doing a huge mlock of the area first?
No, as far as I know, they'll not do huge mlock.

Thanks,
-Kame

> So if two or more are starting up at the same time on the same box,
> more bouncing than is healthy (and more than they would have seen
> in the old days of ZERO_PAGE but no lock_page on it there).
>
> I'd like to persuade myself not to bother,
> but may want to add a further patch for that later.
>
> Hugh
>
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