linux-kernel - Re: [PATCH] tmpfs: use ida to get inode number

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Message-ID: <20191121043127.GA26530@ZenIV.linux.org.uk>
Date:   Thu, 21 Nov 2019 04:31:27 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     zhengbin <zhengbin13@...wei.com>, hughd@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        houtao1@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH] tmpfs: use ida to get inode number

On Wed, Nov 20, 2019 at 07:45:52AM -0800, Matthew Wilcox wrote:
> On Wed, Nov 20, 2019 at 10:23:18PM +0800, zhengbin wrote:
> > I have tried to change last_ino type to unsigned long, while this was
> > rejected, see details on https://patchwork.kernel.org/patch/11023915.
> 
> Did you end up trying sbitmap?
> 
> What I think is fundamentally wrong with this patch is that you've found a
> problem in get_next_ino() and decided to use a different scheme for this
> one filesystem, leaving every other filesystem which uses get_next_ino()
> facing the same problem.
> 
> That could be acceptable if you explained why tmpfs is fundamentally
> different from all the other filesystems that use get_next_ino(), but
> you haven't (and I don't think there is such a difference.  eg pipes,
> autofs and ipc mqueue could all have the same problem.

If you think that anyone is willing to pay one hell of a price on each
pipe(2)...  Note that get_next_ino() is pretty careful about staying
within per-cpu stuff most of the time; it hits any cross-CPU traffic
only in 1/1024th of calls.  This, AFAICS, dirties shared cachelines
on each call.  And there's a plenty of pipe-heavy workloads, for obvious
reasons.