linux-kernel - Re: [PATCH 3/3 v3] mm/vmalloc: Cache the vmalloc memory info

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Message-ID: <20150824010403.27903.qmail@ns.horizon.com>
Date:	23 Aug 2015 21:04:03 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, mingo@...nel.org
Cc:	dave@...1.net, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux@...musvillemoes.dk, peterz@...radead.org, riel@...hat.com,
	rientjes@...gle.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH 3/3 v3] mm/vmalloc: Cache the vmalloc memory info

First, an actual, albeit minor, bug: initializing both vmap_info_gen
and vmap_info_cache_gen to 0 marks the cache as valid, which it's not.

vmap_info_gen should be initialized to 1 to force an initial
cache update.

Second, I don't see why you need a 64-bit counter.  Seqlocks consider
32 bits (31 bits, actually, the lsbit means "update in progress") quite
a strong enough guarantee.

Third, it seems as though vmap_info_cache_gen is basically a duplicate
of vmap_info_lock.sequence.  It should be possible to make one variable
serve both purposes.

You just need a kludge to handle the case of multiple vamp_info updates
between cache updates.

There are two simple ones:

1) Avoid bumping vmap_info_gen unnecessarily.  In vmap_unlock(), do
	vmap_info_gen = (vmap_info_lock.sequence | 1) + 1;
2) - Make vmap_info_gen a seqcount_t
   - In vmap_unlock(), do write_seqcount_barrier(&vmap_info_gen)
   - In get_vmalloc_info, inside the seqlock critical section, do
     vmap_info_lock.seqcount.sequence = vmap_info_gen.sequence - 1;
     (Using the vmap_info_gen.sequence read while validating the
     cache in the first place.)

I should try to write an actual patch illustrating this.
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