linux-kernel - Re: [PATCH 5/11] ksm: get_ksm

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Message-ID: <alpine.LNX.2.00.1301271355430.17144@eggly.anvils>
Date:	Sun, 27 Jan 2013 14:08:00 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Simon Jeons <simon.jeons@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/11] ksm: get_ksm_page locked

On Sat, 26 Jan 2013, Simon Jeons wrote:
> On Fri, 2013-01-25 at 18:00 -0800, Hugh Dickins wrote:
> > In some places where get_ksm_page() is used, we need the page to be locked.
> > 
> 
> In function get_ksm_page, why check page->mapping =>
> get_page_unless_zero => check page->mapping instead of
> get_page_unless_zero => check page->mapping, because
> get_page_unless_zero is expensive?

Yes, it's more expensive.

> 
> > When KSM migration is fully enabled, we shall want that to make sure that
> > the page just acquired cannot be migrated beneath us (raised page count is
> > only effective when there is serialization to make sure migration notices).
> > Whereas when navigating through the stable tree, we certainly do not want
> 
> What's the meaning of "navigating through the stable tree"?

Finding the right place in the stable tree,
as stable_tree_search() and stable_tree_insert() do.

> 
> > to lock each node (raised page count is enough to guarantee the memcmps,
> > even if page is migrated to another node).
> > 
> > Since we're about to add another use case, add the locked argument to
> > get_ksm_page() now.
> 
> Why the parameter lock passed from stable_tree_search/insert is true,
> but remove_rmap_item_from_tree is false?

The other way round?  remove_rmap_item_from_tree needs the page locked,
because it's about to modify the list: that's secured (e.g. against
concurrent KSM page reclaim) by the page lock.

stable_tree_search and stable_tree_insert do not need intermediate nodes
to be locked: get_page is enough to secure the page contents for memcmp,
and we don't want a pointless wait for exclusive page lock on every
intermediate node.
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