Re: [RFC PATCH 3/3] mm: get_first_page_unless_zero()

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Aug 04, 2011 at 02:07:22PM -0700, Michel Lespinasse wrote:
> This change introduces a new get_page_unless_zero() function, to be
> used for idle page tracking in a a future patch series. It also
> illustrates why I care about introducing the page count lock discussed
> in the previous commit.
> 
> To explain the context: for idle page tracking, I am scanning pages
> at a known rate based on their physical address. I want to find out
> if pages have been referenced since the last scan using page_referenced(),
> but before that I must acquire a reference on the page and to basic
> checks about the page type. Before THP, it was safe to acquire references
> using get_page_unless_zero(), but this won't work with in THP enabled kernel
> due to the possible race with __split_huge_page_refcount(). Thus, the new
> proposed get_first_page_unless_zero() function:
> 
> - must act like get_page_unless_zero() if the page is not a tail page;
> - returns 0 for tail pages.
> 
> Without the page count lock I'm proposing, other approaches don't work
> as well to provide mutual exclusion with __split_huge_page_refcount():
> 
> - using the zone LRU lock would work, but has a low granularity and
>   exhibits contention under some of our workloads

I thougt this but it seems your concern is LRU lock contention.

This patch doesn't include any use case(Sometime it hurts reviewers)
but I expect it's in idle tracking patch set.
But we don't conclude yet idle page tracking patchset is reasonable
or not to merge mainline. So, I think it's rather rash idea.
(But I admit [1,2/3] is enough to discuss regardless of idle page tracking)

What I suggestion is as follows,

1. Replace naked page->_count accesses with accessor functions
2. page count lock
3. idle page tracking with simple lock(ex, zone->lru_lock)
4. get_first_page_unless_zero to optimize lock overhead.

-- 
Kind regards,
Minchan Kim

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