linux-kernel - Re: [PATCH 0/3] mm/vmalloc: Cache the /proc/meminfo vmalloc statistics

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Message-ID: <20150823064603.14050.qmail@ns.horizon.com>
Date:	23 Aug 2015 02:46:03 -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 0/3] mm/vmalloc: Cache the /proc/meminfo vmalloc statistics

Ingo Molnar <mingo@...nel.org> wrote:
> I think this is too complex.
> 
> How about something simple like the patch below (on top of the third patch)?

> It makes the vmalloc info transactional - /proc/meminfo will always print a 
> consistent set of numbers. (Not that we really care about races there, but it 
> looks really simple to solve so why not.)

Looks like a huge simplification!

It needs a comment about the approximate nature of the locking and
the obvious race conditions:
1) The first caller to get_vmalloc_info() clears vmap_info_changed
   before updating vmap_info_cache, so a second caller is likely to
   get stale data for the duration of a calc_vmalloc_info call.
2) Although unlikely, it's possible for two threads to race calling
   calc_vmalloc_info, and the one that computes fresher data updates
   the cache first, so the later write leaves stale data.

Other issues:
3) Me, I'd make vmap_info_changed a bool, for documentation more than
   any space saving.
4) I wish there were a trylock version of write_seqlock, so we could
   avoid blocking entirely.  (You *could* hand-roll it, but that eats
   into the simplicity.)
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