On Tue, Feb 07, 2023 at 11:06:13AM +0800, Yin, Fengwei wrote:
> On 2/7/2023 4:34 AM, Matthew Wilcox wrote:
> > On Tue, Jan 24, 2023 at 06:13:21PM +0000, Matthew Wilcox wrote:
> >> Once we get to the part of the folio journey where we have
> >> one-pointer-per-page, we can't afford to maintain per-page state.
> >> Currently we maintain a per-page mapcount, and that will have to go.
> >> We can maintain extra state for a multi-page folio, but it has to be a
> >> constant amount of extra state no matter how many pages are in the folio.
> >>
> >> My proposal is that we maintain a single mapcount per folio, and its
> >> definition is the number of (vma, page table) tuples which have a
> >> reference to any pages in this folio.
> >
> > I've been thinking about this a lot more, and I have changed my
> > mind. It works fine to answer the question "Is any page in this
> > folio mapped", but it's now hard to answer the question "I have it
> > mapped, does anybody else?" That question is asked, for example,
> > in madvise_cold_or_pageout_pte_range().
> >
> > With this definition, if the mapcount is 1, it's definitely only mapped
> > by us. If it's more than 2, it's definitely mapped by somebody else (*).
> > If it's 2, maybe we have the folio mapped twice, and maybe we have it
> > mapped once and somebody else has it mapped once, so we have to consult
> > the rmap to find out. Not fun times.
> Naive question: If it's 2, why do we need to know whether it's mapped twice
> by us or one by us and one by somebody else? Does that mean we allow some
> operations if only us mapped it even it's 2? Thanks.
Yes, see the aforementioned madvise_cold_or_pageout_pte_range():
(in mainline, it's different in -next):
/* Do not interfere with other mappings of this page */
if (page_mapcount(page) != 1)
goto huge_unlock;
