From: Erik Jensen <erikjensen@rkjnsn.net>
To: Matthew Wilcox <willy@infradead.org>, Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: Linux FS Devel <linux-fsdevel@vger.kernel.org>,
"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: page->index limitation on 32bit system?
Date: Sat, 20 Feb 2021 15:02:26 -0800 [thread overview]
Message-ID: <b3e40749-a30d-521a-904f-8182c6d0e258@rkjnsn.net> (raw)
In-Reply-To: <20210218133954.GR2858050@casper.infradead.org>
On 2/18/21 5:39 AM, Matthew Wilcox wrote:
> On Thu, Feb 18, 2021 at 08:42:14PM +0800, Qu Wenruo wrote:
>> [...]
>> BTW, what would be the extra cost by converting page::index to u64?
>> I know tons of printk() would cause warning, but most 64bit systems
>> should not be affected anyway.
>
> No effect for 64-bit systems, other than the churn.
>
> For 32-bit systems, it'd have some pretty horrible overhead. You don't
> just have to touch the page cache, you have to convert the XArray.
> It's doable (I mean, it's been done), but it's very costly for all the
> 32-bit systems which don't use a humongous filesystem. And we could
> minimise that overhead with a typedef, but then the source code gets
> harder to work with.
Out of curiosity, would it be at all feasible to use 64-bits for the
page offset *without* changing XArray, perhaps by indexing by the lower
32-bits, and evicting the page that's there if the top bits don't match
(vaguely like how the CPU cache works)? Or, if there are cases where a
page can't be evicted (I don't know if this can ever happen), use chaining?
I would expect index contention to be extremely uncommon, and it could
only happen for inodes larger than 16 TiB, which can't be used at all
today. I don't know how many data structures store page offsets today,
but it seems like this should significantly reduce the performance
impact versus upping XArray to 64-bit indexes.
next prev parent reply other threads:[~2021-02-20 23:03 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 8:54 page->index limitation on 32bit system? Qu Wenruo
2021-02-18 12:15 ` Matthew Wilcox
2021-02-18 12:42 ` Qu Wenruo
2021-02-18 13:39 ` Matthew Wilcox
2021-02-19 0:37 ` Qu Wenruo
2021-02-19 16:12 ` Theodore Ts'o
2021-02-19 23:10 ` Qu Wenruo
2021-02-20 0:23 ` Matthew Wilcox
2021-02-22 0:19 ` Dave Chinner
2021-02-20 2:20 ` Erik Jensen
2021-02-20 3:40 ` Matthew Wilcox
2021-02-20 23:02 ` Erik Jensen [this message]
2021-02-20 23:22 ` Matthew Wilcox
2021-02-21 0:01 ` Erik Jensen
2021-02-21 17:15 ` Matthew Wilcox
2021-02-18 21:27 ` Erik Jensen
2021-02-19 14:22 ` Matthew Wilcox
2021-02-19 17:51 ` Matthew Wilcox
2021-02-19 23:13 ` Qu Wenruo
2021-02-22 1:48 ` Dave Chinner
2021-03-01 1:49 ` GWB
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