netdev - Re: [PATCH 3/3] rhashtable: Add nested tables

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Date:   Tue, 7 Feb 2017 14:17:28 +0100
From:   Florian Westphal <fw@...len.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/3] rhashtable: Add nested tables

Herbert Xu <herbert@...dor.apana.org.au> wrote:
> This patch adds code that handles GFP_ATOMIC kmalloc failure on
> insertion.  As we cannot use vmalloc, we solve it by making our
> hash table nested.  That is, we allocate single pages at each level
> and reach our desired table size by nesting them.
> 
> When a nested table is created, only a single page is allocated
> at the top-level.  Lower levels are allocated on demand during
> insertion.  Therefore for each insertion to succeed, only two
> (non-consecutive) pages are needed.
> 
> After a nested table is created, a rehash will be scheduled in
> order to switch to a vmalloced table as soon as possible.  Also,
> the rehash code will never rehash into a nested table.  If we
> detect a nested table during a rehash, the rehash will be aborted
> and a new rehash will be scheduled.

Ok, but why?

It seems to add a whole lot of complexity...

What users can't handle the insert failure case until resize
has completed?

Would relaxing the max chain length (until rehash is done) be an
alternative?