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From:   "J. R. Okajima" <hooanon05g@gmail.com>
Subject: Re: [PATCH] tmpfs: use ida to get inode number
To:     Hugh Dickins <hughd@google.com>
Cc:     "zhengbin (A)" <zhengbin13@huawei.com>,
        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
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Date:   Thu, 21 Nov 2019 20:40:26 +0900
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Hugh Dickins:
> 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,
>
> =09ino =3D sbinfo->next_ino++;
> =09/* Avoid 0 in the low 32 bits: might appear deleted */
> =09if (unlikely((unsigned int)ino =3D=3D 0))
> =09=09ino =3D sbinfo->next_ino++;

I agree with that "per superblock inum space", but I don't see your
point.  How can you manage it fully?  I mean how can you decide whether
the new inum is in use or not?
For example,
- you create a file which is assigned inum#10.
- you or other people create and unlink over and over on the same tmpfs.
- then sbinfo->next_ino will become zero, skipped, ok.
- and then it will be 10.
I don't think you want to share the same inum by two inodes.

Moreover, SysV SHM uses tmpfs and shmget(2) overwrite inum internally.
It will be another seed of a similar problem.


J. R. Okajima