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Subject: Re: [PATCH] tmpfs: use ida to get inode number
To:     Hugh Dickins <hughd@google.com>
CC:     Matthew Wilcox <willy@infradead.org>, <viro@zeniv.linux.org.uk>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <houtao1@huawei.com>, <yi.zhang@huawei.com>,
        "J. R. Okajima" <hooanon05g@gmail.com>
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 <alpine.LSU.2.11.1911211125190.1697@eggly.anvils>
From:   "zhengbin (A)" <zhengbin13@huawei.com>
Message-ID: <5423a199-eefb-0a02-6e86-1f6210939c11@huawei.com>
Date:   Fri, 22 Nov 2019 09:23:30 +0800
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On 2019/11/22 3:53, Hugh Dickins wrote:
> On Thu, 21 Nov 2019, zhengbin (A) wrote:
>> On 2019/11/21 12:52, Hugh Dickins wrote:
>>> Just a rushed FYI without looking at your patch or comments.
>>>
>>> Internally (in Google) we do rely on good tmpfs inode numbers more
>>> than on those of other get_next_ino() filesystems, and carry a patch
>>> to mm/shmem.c for it to use 64-bit inode numbers (and separate inode
>>> number space for each superblock) - essentially,
>>>
>>> 	ino = sbinfo->next_ino++;
>>> 	/* Avoid 0 in the low 32 bits: might appear deleted */
>>> 	if (unlikely((unsigned int)ino == 0))
>>> 		ino = sbinfo->next_ino++;
>>>
>>> Which I think would be faster, and need less memory, than IDA.
>>> But whether that is of general interest, or of interest to you,
>>> depends upon how prevalent 32-bit executables built without
>>> __FILE_OFFSET_BITS=64 still are these days.
>> So how google think about this? inode number > 32-bit, but 32-bit executables
>> cat not handle this?
> Google is free to limit what executables are run on its machines,
> and how they are built, so little problem here.
>
> A general-purpose 32-bit Linux distribution does not have that freedom,
> does not want to limit what the user runs.  But I thought that by now
> they (and all serious users of 32-bit systems) were building their own
> executables with _FILE_OFFSET_BITS=64 (I was too generous with the
> underscores yesterday); and I thought that defined __USE_FILE_OFFSET64,
> and that typedef'd ino_t to be __ino64_t.  And the 32-bit kernel would
> have __ARCH_WANT_STAT64, which delivers st_ino as unsigned long long.
>
> So I thought that a modern, professional 32-bit executable would be
> dealing in 64-bit inode numbers anyway.  But I am not a system builder,
> so perhaps I'm being naive.  And of course some users may have to support
> some old userspace, or apps that assign inode numbers to "int" or "long"
> or whatever.  I have no insight into the extent of that problem.

So how to solve this problem?

1. tmpfs use ida or other data structure

2. tmpfs use 64-bit, each superblock a inode number space

3. do not do anything, If somebody hits this bug, let them solve for themselves

4. (last_ino change to 64-bit)get_next_ino -->other filesystems will be ok, but it was rejected before

>
> Hugh
>
> .
>