Re: [PATCH v3] mm: Fix slab->page flags corruption.

[Andrea Arcangeli]

On Thu, May 17, 2012 at 03:17:49PM -0700, Pravin B Shelar wrote:
> diff --git a/mm/swap.c b/mm/swap.c
> index 8ff73d8..44a0f81 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -82,6 +82,19 @@ static void put_compound_page(struct page *page)
>  		if (likely(page != page_head &&
>  			   get_page_unless_zero(page_head))) {
>  			unsigned long flags;
> +
> +			if (PageSlab(page_head)) {
> +				if (PageTail(page)) {
> +					/* THP can not break up slab pages, avoid
> +					 * taking compound_lock(). */
> +					if (put_page_testzero(page_head))
> +						VM_BUG_ON(1);
> +
> +					atomic_dec(&page->_mapcount);
> +					goto skip_lock_tail;
> +				} else
> +					goto skip_lock;
> +			}

Some commentary on the fact slab prefers not using atomic ops on the
page->flags could help here.

>  			/*
>  			 * page_head wasn't a dangling pointer but it
>  			 * may not be a head page anymore by the time
> @@ -93,6 +106,7 @@ static void put_compound_page(struct page *page)
>  				/* __split_huge_page_refcount run before us */
>  				compound_unlock_irqrestore(page_head, flags);
>  				VM_BUG_ON(PageHead(page_head));

Hmmm hmmm while reviewing this one, I've been thinking maybe the head
page after the hugepage split, could have been freed and reallocated
as order 1 or 2, and legitimately become an head page again.

The whole point of the bug-on is that it cannot be reallocated as a
THP beause the tail is still there and it's not free yet, but it
doesn't take into account the head page could be allocated as a
compound page of a smaller size and maybe the tail is the last subpage
of the thp.

So there's the risk of a false positive, in an extremely unlikely case
(the fact slab goes in unmovable pageblocks and thp goes in movable
further decreases the probability). All production kernels runs with
VM_BUG_ON disabled so it's a very small concern, but maybe we should
delete it. It has never triggered, just code reivew. Do you agree?

> +			skip_lock:
>  				if (put_page_testzero(page_head))
>  					__put_single_page(page_head);
>  			out_put_single:
> @@ -115,6 +129,8 @@ static void put_compound_page(struct page *page)
>  			VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
>  			VM_BUG_ON(atomic_read(&page->_count) != 0);
>  			compound_unlock_irqrestore(page_head, flags);
> +
> +			skip_lock_tail:
>  			if (put_page_testzero(page_head)) {
>  				if (PageHead(page_head))
>  					__put_compound_page(page_head);
> @@ -162,6 +178,15 @@ bool __get_page_tail(struct page *page)
>  	struct page *page_head = compound_trans_head(page);
>  
>  	if (likely(page != page_head && get_page_unless_zero(page_head))) {
> +
> +		if (PageSlab(page_head)) {
> +			if (likely(PageTail(page))) {
> +				__get_page_tail_foll(page, false);
> +				return true;
> +			} else
> +				goto out;
> +		}
> +

A comment here too would be nice.

>  		/*
>  		 * page_head wasn't a dangling pointer but it
>  		 * may not be a head page anymore by the time
> @@ -175,6 +200,8 @@ bool __get_page_tail(struct page *page)
>  			got = true;
>  		}
>  		compound_unlock_irqrestore(page_head, flags);
> +
> +		out:
>  		if (unlikely(!got))
>  			put_page(page_head);

out could go in the line below. Assuming we don't want to be cleaner
and use put_page above instead of goto, that would also drop a branch
probably (the goto place is such a slow path). I'm fine either ways.

It's not the cleanest of the patches but it's clearly a performance
tweak.

Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Thanks,
Andrea
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