On Thu, 8 Mar 2012 23:25:28 -0500
Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:
> Hi KAMEZAWA-san,
>
> > On Fri, 2 Mar 2012 15:13:09 -0500
> > Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:
> ...
> > > +
> > > + page = pmd_page(pmd);
> > > + VM_BUG_ON(!page || !PageHead(page));
> > > + if (!move_anon() || page_mapcount(page) != 1)
> > > + return 0;
> >
> > Could you add this ?
> > ==
> > static bool move_check_shared_map(struct page *page)
> > {
> > /*
> > * Handling of shared pages between processes is a big trouble in memcg.
> > * Now, we never move shared-mapped pages between memcg at 'task' moving because
> > * we have no hint which task the page is really belongs to. For example,
> > * When a task does "fork()-> move to the child other group -> exec()", the charges
> > * should be stay in the original cgroup.
> > * So, check mapcount to determine we can move or not.
> > */
> > return page_mapcount(page) != 1;
> > }
> > ==
>
> Thank you.
>
> We check mapcount only for anonymous pages, so we had better also describe
> that viewpoint? And this function returns whether the target page of moving
> charge is shared or not, so a name like is_mctgt_shared() looks better to me.
> Moreover, this function explains why we have current implementation, rather
> than why return value is mapcount != 1, so I put the comment above function
> declaration like this:
>
> /*
> * Handling of shared pages between processes is a big trouble in memcg.
> * Now, we never move shared anonymous pages between memcg at 'task'
> * moving because we have no hint which task the page is really belongs to.
> * For example, when a task does "fork() -> move to the child other group
> * -> exec()", the charges should be stay in the original cgroup.
> * So, check if a given page is shared or not to determine to move charge.
> */
> static bool is_mctgt_shared(struct page *page)
> {
> return page_mapcount(page) != 1;
> }
>
> As for the difference between anon page and filemapped page, I have no idea
> about current charge moving policy. Is this explained anywhere? (sorry to
> question before researching by myself ...)
>
>
Now, I think it's okay to move mapcount check. I posted a patch for reference.
Please check it.
https://lkml.org/lkml/2012/3/9/40
> > We may be able to support madvise(MOVE_MEMCG) or fadvise(MOVE_MEMCG), if necessary.
>
> Is this mean moving charge policy can depend on users?
> I feel that's strange because I don't think resouce management should be
> under users' control.
>
You're right. I
Hm. I remember some guy suggested 'how about passing prefer memcg as mount option'
or some. Anyway, shared page handling is trouble since memory cgroup was born.
Thanks,
-Kame
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