Re: [RFC][PATCH] memcg: avoid THP split in task migration

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Naoya,

On Tue, Feb 28, 2012 at 04:12:32PM -0500, Naoya Horiguchi wrote:
> Currently we can't do task migration among memory cgroups without THP split,
> which means processes heavily using THP experience large overhead in task
> migration. This patch introduce the code for moving charge of THP and makes
> THP more valuable.

Nice.

> diff --git linux-next-20120228.orig/mm/memcontrol.c linux-next-20120228/mm/memcontrol.c
> index c83aeb5..e97c041 100644
> --- linux-next-20120228.orig/mm/memcontrol.c
> +++ linux-next-20120228/mm/memcontrol.c
> @@ -5211,6 +5211,42 @@ static int is_target_pte_for_mc(struct vm_area_struct *vma,
>  	return ret;
>  }
>  
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +/*
> + * We don't consider swapping or file mapped pages because THP does not
> + * support them for now.
> + */
> +static int is_target_huge_pmd_for_mc(struct vm_area_struct *vma,
> +		unsigned long addr, pmd_t pmd, union mc_target *target)
> +{
> +	struct page *page = NULL;
> +	struct page_cgroup *pc;
> +	int ret = 0;
> +
> +	if (pmd_present(pmd))
> +		page = pmd_page(pmd);
> +	if (!page)
> +		return 0;

It can't be present and null at the same time.

No need to check pmd_present if you already checked pmd_trans_huge. In
fact checking pmd_present is a bug. For a little time the pmd won't be
present if it's set as splitting. (that short clearing of pmd_present
during pmd splitting is to deal with a vendor CPU errata without
having to flush the smp TLB twice)

Following Kame's suggestion is correct, an unconditional pmd_page is
correct here:

    page = pmd_page(pmd);

We might actually decide to change pmd_present to return true if
pmd_trans_splitting is set to avoid the risk of using an erratic
pmd_present on a pmd_trans_huge pmd, but it's not really necessary if
you never check pmd_present when a pmd is (or can be) a
pmd_trans_huge.

The safe check for pmd is only pmd_none, never pmd_present (as in
__pte_alloc/pte_alloc_map/...).

> +	VM_BUG_ON(!PageHead(page));
> +	get_page(page);

Other review mentioned we can do get_page only when it succeeds, but I
think we can drop the whole get_page and simplify it further see the
end.

> @@ -5219,7 +5255,13 @@ static int mem_cgroup_count_precharge_pte_range(pmd_t *pmd,
>  	pte_t *pte;
>  	spinlock_t *ptl;
>  
> -	split_huge_page_pmd(walk->mm, pmd);
> +	if (pmd_trans_huge_lock(pmd, vma) == 1) {
> +		if (is_target_huge_pmd_for_mc(vma, addr, *pmd, NULL))
> +			mc.precharge += HPAGE_PMD_NR;

Your use of HPAGE_PMD_NR looks fine, that path will be eliminated at
build time if THP is off. This is the nice way to write code that is
already optimal for THP=off without making special cases or #ifdefs.

Other review suggests changing HPAGE_PMD_NR as BUILD_BUG, that sounds
good idea too, but in this (correct) usage of HPAGE_PMD_NR it wouldn't
make a difference because of the whole branch is correctly eliminated
at build time. In short changing it to BUILD_BUG will simply make sure
the whole pmd_trans_huge_lock == 1 branch is eliminated at build
time. It looks good change too but it's orthogonal so I'd leave it for
a separate patch.

> +		spin_unlock(&walk->mm->page_table_lock);

Agree with other review, vma looks cleaner.

> +		cond_resched();
> +		return 0;
> +	}
>  
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE)
> @@ -5378,16 +5420,38 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
>  	struct vm_area_struct *vma = walk->private;
>  	pte_t *pte;
>  	spinlock_t *ptl;
> +	int type;
> +	union mc_target target;
> +	struct page *page;
> +	struct page_cgroup *pc;
> +
> +	if (pmd_trans_huge_lock(pmd, vma) == 1) {
> +		if (!mc.precharge)
> +			return 0;

Agree with Hillf.

> +		type = is_target_huge_pmd_for_mc(vma, addr, *pmd, &target);
> +		if (type == MC_TARGET_PAGE) {
> +			page = target.page;
> +			if (!isolate_lru_page(page)) {
> +				pc = lookup_page_cgroup(page);
> +				if (!mem_cgroup_move_account(page, HPAGE_PMD_NR,
> +							     pc, mc.from, mc.to,
> +							     false)) {
> +					mc.precharge -= HPAGE_PMD_NR;
> +					mc.moved_charge += HPAGE_PMD_NR;
> +				}

Like you mentioned, a race with split_huge_page_refcount (and hence
mem_cgroup_split_huge_fixup) is not possible because of
pmd_trans_huge_lock succeeding.

However the mmap_sem checked by pmd_trans_huge_lock is there just
because we deal with pmds and so pagetables (and we aren't doing a
lockless lookup like gup_fast). But it's not true that a concurrent
split_huge_page would _not_ prevented by the mmap_sem. The swapout
path will still split hugepages under you even if you hold the
mmap_sem (even in write mode).

The mmap_sem (either read or write) only prevents a concurrent
collapsing/creation of hugepages (but that's irrelevant here). It
won't stop split_huge_page.

So - back to our issue - you're safe against split_huge_page not
running here thanks to the pmd_trans_huge_lock.

There's one tricky locking bit here, that is isolate_lru_page, that
takes the lru_lock.

So the lock order is the page_table_lock first and the lru_lock
second, and so there must not be another place that takes the lru_lock
first and the page_table_lock second. In general it's good idea to
exercise locking code with lockdep prove locking enabled just in case.

> +				putback_lru_page(page);
> +			}
> +			put_page(page);

I wonder if you need a get_page at all in is_target_huge_pmd_for_mc if
you drop the above put_page instead. How can this page go away from
under us, if we've been holding the page_table_lock the whole time?
You can probably drop both get_page above and put_page above.

> +		}
> +		spin_unlock(&walk->mm->page_table_lock);
> +		cond_resched();
> +		return 0;
> +	}
>  
> -	split_huge_page_pmd(walk->mm, pmd);
>  retry:
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; addr += PAGE_SIZE) {
>  		pte_t ptent = *(pte++);
> -		union mc_target target;
> -		int type;
> -		struct page *page;
> -		struct page_cgroup *pc;
>  		swp_entry_t ent;
>  
>  		if (!mc.precharge)

I read the other two great reviews done so far in parallel with the
code, and I ended up replying here to the code as I was reading it,
hope it wasn't too confusing.

Thanks!
Andrea

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