linux-kernel - Re: [V9fs-developer] [PATCH v2 4/6] 9p: Embed wait_queue_head into p9_req

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Date:   Thu, 12 Jul 2018 16:40:12 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Greg Kurz <groug@...d.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        Eric Van Hensbergen <ericvh@...il.com>,
        linux-kernel@...r.kernel.org, Ron Minnich <rminnich@...dia.gov>,
        linux-fsdevel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net
Subject: Re: [V9fs-developer] [PATCH v2 4/6] 9p: Embed wait_queue_head into
 p9_req_t

Greg Kurz wrote on Thu, Jul 12, 2018:
> This is true when all tags have been used at least once. But the current code
> lazily allocates the wait_queue_head_t, ie, only when a tag is used for the
> first time. Your patch causes a full row of wait_quest_head_t to be pre-allocated.
> 
> ie, P9_ROW_MAXTAG * 24 = 255 * 24 = 6120
> 
> instead of (P9_ROW_MAXTAG * 8) + 24 = 255 * 8 + 24 = 2064
> 
> This is nearly a page of allocated memory that might be never used.
> 
> Not sure if this is a problem though...

I thought the exact same, but the next patch in the serie removes that
array and allocates even more lazily with a slab, so this was a
no-brainer :)

-- 
Dominique Martinet