Re: [PATCH RFC 00/15] mm: memory book keeping and lru_lock splitting

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Hugh Dickins wrote:
> On Thu, 16 Feb 2012, Hugh Dickins wrote:
>>
>> Yours are not the only patches I was testing in that tree, I tried to
>> gather several other series which I should be reviewing if I ever have
>> time: Kamezawa-san's page cgroup diet 6, Xiao Guangrong's 4 prio_tree
>> cleanups, your 3 radix_tree changes, your 6 shmem changes, your 4 memcg
>> miscellaneous, and then your 15 books.
>>
>> The tree before your final 15 did well under pressure, until I tried to
>> rmdir one of the cgroups afterwards: then it crashed nastily, I'll have
>> to bisect into that, probably either Kamezawa's or your memcg changes.
>
> So far I haven't succeeded in reproducing that at all: it was real,
> but obviously harder to get than I assumed - indeed, no good reason
> to associate it with any of those patches, might even be in 3.3-rc.
>
> It did involve a NULL pointer dereference in mem_cgroup_page_lruvec(),
> somewhere below compact_zone() - but repercussions were causing the
> stacktrace to scroll offscreen, so I didn't get good details.

There some stupid bugs in my v1 patchset, it shouldn't works at all.
I did not expect that someone will try to use it. I sent it just to discuss.

Most destructive bug is this PageCgroupUsed() below:

+struct book *page_book(struct page *page)
+{
+	struct mem_cgroup_per_zone *mz;
+	struct page_cgroup *pc;
+
+	if (mem_cgroup_disabled())
+		return &page_zone(page)->book;
+
+	pc = lookup_page_cgroup(page);
+	if (!PageCgroupUsed(pc))
+		return &page_zone(page)->book;
+	/* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
+	smp_rmb();
+	mz = mem_cgroup_zoneinfo(pc->mem_cgroup,
+			page_to_nid(page), page_zonenum(page));
+	return &mz->book;
+}

Thus after page uncharge I remove page from wrong book, under wrong lock =)

[ as I wrote, updated patchset there: https://github.com/koct9i/linux ]

>
> Hugh

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