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

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Date:   Thu, 21 Nov 2019 10:36:05 +0800
From:   "zhengbin (A)" <zhengbin13@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <viro@...iv.linux.org.uk>, <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 2019/11/20 23:45, 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?

Maybe sbitmap is not a good solution, max_inodes of tmpfs are controlled by mount options--nrinodes,

which can be modified by remountfs(bigger or smaller), as the comment of function sbitmap_resize says:

 * Doesn't reallocate anything. It's up to the caller to ensure that the new
 * depth doesn't exceed the depth that the sb was initialized with.

We can modify this to meet the growing requirements, there will still be questions as follows:

1. tmpfs is a ram filesystem, we need to allocate sbitmap memory for sbinfo->max_inodes(while this maybe huge)

2.If remountfs changes  max_inode, we have to deal with it, while this may take a long time

(bigger: we need to free the old sbitmap memory, allocate new memory, copy the old sbitmap to new sbitmap

smaller: How do we deal with it?ie: we use sb->map[inode number/8] to find the sbitmap, we need to change the exist

inode numbers?while this maybe used by userspace application.)

>
> 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.

tmpfs is same with all the other filesystems that use get_next_ino(), but we need to solve this problem one by one.

If tmpfs is ok, we can modify the other filesystems too. Besides, I do not  recommend all file systems share the same

global variable, for performance impact consideration.

>
> There are some other problems I noticed, but they're not worth bringing
> up until this fundamental design choice is justified.
Agree, thanks.
>