Re: [PATCH 1/2] mm: drop migrate type checks from has_unmovable_pages

[Joonsoo Kim]

On Fri, Oct 20, 2017 at 09:02:20AM +0200, Michal Hocko wrote:
> On Fri 20-10-17 15:50:14, Joonsoo Kim wrote:
> > On Fri, Oct 20, 2017 at 07:59:22AM +0200, Michal Hocko wrote:
> > > On Fri 20-10-17 11:13:29, Joonsoo Kim wrote:
> > > > On Thu, Oct 19, 2017 at 02:21:18PM +0200, Michal Hocko wrote:
> > > > > On Thu 19-10-17 10:20:41, Michal Hocko wrote:
> > > > > > On Thu 19-10-17 16:33:56, Joonsoo Kim wrote:
> > > > > > > On Thu, Oct 19, 2017 at 09:15:03AM +0200, Michal Hocko wrote:
> > > > > > > > On Thu 19-10-17 11:51:11, Joonsoo Kim wrote:
> > > > > > [...]
> > > > > > > > > Hello,
> > > > > > > > > 
> > > > > > > > > This patch will break the CMA user. As you mentioned, CMA allocation
> > > > > > > > > itself isn't migrateable. So, after a single page is allocated through
> > > > > > > > > CMA allocation, has_unmovable_pages() will return true for this
> > > > > > > > > pageblock. Then, futher CMA allocation request to this pageblock will
> > > > > > > > > fail because it requires isolating the pageblock.
> > > > > > > > 
> > > > > > > > Hmm, does this mean that the CMA allocation path depends on
> > > > > > > > has_unmovable_pages to return false here even though the memory is not
> > > > > > > > movable? This sounds really strange to me and kind of abuse of this
> > > > > > > 
> > > > > > > Your understanding is correct. Perhaps, abuse or wrong function name.
> > > > > > >
> > > > > > > > function. Which path is that? Can we do the migrate type test theres?
> > > > > > > 
> > > > > > > alloc_contig_range() -> start_isolate_page_range() ->
> > > > > > > set_migratetype_isolate() -> has_unmovable_pages()
> > > > > > 
> > > > > > I see. It seems that the CMA and memory hotplug have a very different
> > > > > > view on what should happen during isolation.
> > > > > >  
> > > > > > > We can add one argument, 'XXX' to set_migratetype_isolate() and change
> > > > > > > it to check migrate type rather than has_unmovable_pages() if 'XXX' is
> > > > > > > specified.
> > > > > > 
> > > > > > Can we use the migratetype argument and do the special thing for
> > > > > > MIGRATE_CMA? Like the following diff?
> > > > > 
> > > > > And with the full changelog.
> > > > > ---
> > > > > >From 8cbd811d741f5dd93d1b21bb3ef94482a4d0bd32 Mon Sep 17 00:00:00 2001
> > > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > > > Date: Thu, 19 Oct 2017 14:14:02 +0200
> > > > > Subject: [PATCH] mm: distinguish CMA and MOVABLE isolation in
> > > > >  has_unmovable_pages
> > > > > 
> > > > > Joonsoo has noticed that "mm: drop migrate type checks from
> > > > > has_unmovable_pages" would break CMA allocator because it relies on
> > > > > has_unmovable_pages returning false even for CMA pageblocks which in
> > > > > fact don't have to be movable:
> > > > > alloc_contig_range
> > > > >   start_isolate_page_range
> > > > >     set_migratetype_isolate
> > > > >       has_unmovable_pages
> > > > > 
> > > > > This is a result of the code sharing between CMA and memory hotplug
> > > > > while each one has a different idea of what has_unmovable_pages should
> > > > > return. This is unfortunate but fixing it properly would require a lot
> > > > > of code duplication.
> > > > > 
> > > > > Fix the issue by introducing the requested migrate type argument
> > > > > and special case MIGRATE_CMA case where CMA page blocks are handled
> > > > > properly. This will work for memory hotplug because it requires
> > > > > MIGRATE_MOVABLE.
> > > > 
> > > > Unfortunately, alloc_contig_range() can be called with
> > > > MIGRATE_MOVABLE so this patch cannot perfectly fix the problem.
> > > 
> > > Yes, alloc_contig_range can be called with MIGRATE_MOVABLE but my
> > > understanding is that only CMA allocator really depends on this weird
> > > semantic and that does MIGRATE_CMA unconditionally.
> > 
> > alloc_contig_range() could be called for partial pages in the
> > pageblock. With your patch, this case also fails unnecessarilly if the
> > other pages in the pageblock is pinned.
> 
> Is this really the case for GB pages? Do we really want to mess those

No, but, as I mentioned already, this API can be called with less
pages. I know that there is no user with less pages at this moment but
I cannot see any point to reduce this API's capability.

> with CMA blocks and make those blocks basically unusable because GB
> pages are rarely (if at all migrateable)?
> 
> > Until now, there is no user calling alloc_contig_range() with partial
> > pages except CMA allocator but API could support it.
> 
> I disagree. If this is a CMA thing it should stay that way. The semantic
> is quite confusing already, please let's not make it even worse.

It is already used by other component.

I'm not sure what is the confusing semantic you mentioned. I think
that set_migratetype_isolate() has confusing semantic and should be
fixed since making the pageblock isolated doesn't need to check if
there is unmovable page or not. Do you think that
set_migratetype_isolate() need to check it? If so, why?

> > > > I did a more thinking and found that it's strange to check if there is
> > > > unmovable page in the pageblock during the set_migratetype_isolate().
> > > > set_migratetype_isolate() should be just for setting the migratetype
> > > > of the pageblock. Checking other things should be done by another
> > > > place, for example, before calling the start_isolate_page_range() in
> > > > __offline_pages().
> > > 
> > > How do we guarantee the atomicity?
> > 
> > What atomicity do you mean?
> 
> Currently we are checking and isolating pages under zone lock. If we
> split that we are losing atomicity, aren't we.

I think that it can be done easily.

set_migratetype_isolate() {
        lock
        __set_migratetype_isolate();
        unlock
}

set_migratetype_isolate_if_no_unmovable_pages() {
        lock
        if (has_unmovable_pages())
                fail
        else
                __set_migratetype_isolate()
        unlock
}

Thanks.