linux-kernel - Re: [PATCH 1/1] vmalloc: purge_fragmented_blocks: Acquire spinlock before reading vmap

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Message-ID: <CAFPAmTSKXCiXNnFK0zR651ONju+ZBYE0qWUhCF9GXZRy=ieSJw@mail.gmail.com>
Date:	Thu, 8 Dec 2011 12:56:22 +0530
From:	Kautuk Consul <consul.kautuk@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Minchan Kim <minchan.kim@...il.com>,
	David Vrabel <david.vrabel@...rix.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] vmalloc: purge_fragmented_blocks: Acquire spinlock
 before reading vmap_block

>
> That's intentional as an optimization, we don't care if
> vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS
> would speculatively be true after we grab vb->lock, we'll have to purge it
> next time instead.  We certainly don't want to grab vb->lock for blocks
> that aren't candidates, so this optimization is a singificant speedup.

Ah, I agree.
Anyway, the probability of there being too many vmap_blocks being
missed due to concurrent changes
is not quite high, so I guess its okay that a few vmap_blocks get
purged next time.

Thanks.
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