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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Thu, 18 Feb 2021 07:52:25 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, cgoldswo@codeaurora.org,
        linux-fsdevel@vger.kernel.org, david@redhat.com, vbabka@suse.cz,
        viro@zeniv.linux.org.uk, joaodias@google.com
Subject: Re: [RFC 1/2] mm: disable LRU pagevec during the migration
 temporarily
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On Thu, Feb 18, 2021 at 09:17:02AM +0100, Michal Hocko wrote:
> On Wed 17-02-21 13:32:05, Minchan Kim wrote:
> > On Wed, Feb 17, 2021 at 09:16:12PM +0000, Matthew Wilcox wrote:
> > > On Wed, Feb 17, 2021 at 12:46:19PM -0800, Minchan Kim wrote:
> > > > > I suspect you do not want to add atomic_read inside hot paths, right? Is
> > > > > this really something that we have to microoptimize for? atomic_read is
> > > > > a simple READ_ONCE on many archs.
> > > > 
> > > > It's also spin_lock_irq_save in some arch. If the new synchonization is
> > > > heavily compilcated, atomic would be better for simple start but I thought
> > > > this locking scheme is too simple so no need to add atomic operation in
> > > > readside.
> > > 
> > > What arch uses a spinlock for atomic_read()?  I just had a quick grep and
> > > didn't see any.
> > 
> > Ah, my bad. I was confused with update side.
> > Okay, let's use atomic op to make it simple.
> 
> Thanks. This should make the code much more simple. Before you send
> another version for the review I have another thing to consider. You are
> kind of wiring this into the migration code but control over lru pcp
> caches can be used in other paths as well. Memory offlining would be
> another user. We already disable page allocator pcp caches to prevent
> regular draining. We could do the same with lru pcp caches.

I didn't catch your point here. If memory offlining is interested on
disabling lru pcp, it could call migrate_prep and migrate_finish
like other places. Are you suggesting this one?

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a969463bdda4..0ec1c13bfe32 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1425,8 +1425,12 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
                node_clear(mtc.nid, nmask);
                if (nodes_empty(nmask))
                        node_set(mtc.nid, nmask);
+
+               migrate_prep();
                ret = migrate_pages(&source, alloc_migration_target, NULL,
                        (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
+
+               migrate_finish();
                if (ret) {
                        list_for_each_entry(page, &source, lru) {
                                pr_warn("migrating pfn %lx failed ret:%d ",