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SubjectRe: [PATCH v7 2/2] tmpfs: Support 64-bit inums per-sb
On Mon, 13 Jul 2020, Chris Down wrote:

> The default is still set to inode32 for backwards compatibility, but
> system administrators can opt in to the new 64-bit inode numbers by
> either:
>
> 1. Passing inode64 on the command line when mounting, or
> 2. Configuring the kernel with CONFIG_TMPFS_INODE64=y
>
> The inode64 and inode32 names are used based on existing precedent from
> XFS.
>
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Hugh Dickins <hughd@google.com>

Again, thanks, and comments below, but nothing to override tha Ack.

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: linux-mm@kvack.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-team@fb.com
> ---
> Documentation/filesystems/tmpfs.rst | 11 +++++
> fs/Kconfig | 15 +++++++
> include/linux/shmem_fs.h | 1 +
> mm/shmem.c | 65 ++++++++++++++++++++++++++++-
> 4 files changed, 90 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
> index 4e95929301a5..47b84ddaa8bb 100644
> --- a/Documentation/filesystems/tmpfs.rst
> +++ b/Documentation/filesystems/tmpfs.rst
> @@ -150,6 +150,15 @@ These options do not have any effect on remount. You can change these
> parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
>
>
> +tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
> +numbers:
> +
> +inode64 Use 64-bit inode numbers
> +inode32 Use 32-bit inode numbers
> +
> +On 64-bit, the default is set by CONFIG_TMPFS_INODE64. On 32-bit, inode64 is
> +not legal and will produce an error at mount time.
> +

I did originally want to move this up higher, between the discussion
of nr_inodes and mpol; but its placing here follows where you placed
it in /proc/mounts, and I end up agreeing with that, so let's leave
where you've placed it.

But this is a description of a new pair of mount options, nothing to
do with the paragraph below, so one more blank line is needed.

Except that tmpfs.txt has now been replaced by tmpfs.rst, with some
====ing too. We'd better add that. I'm unfamiliar with, and not
prepared for .rst - getting Doc right has become a job for specialists.
I'll send a -fix.patch of how how I think it should look to Andrew and
Randy Dunlap, hoping Randy can confirm whether it ends up right.

> So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
> will give you tmpfs instance on /mytmpfs which can allocate 10GB
> RAM/SWAP in 10240 inodes and it is only accessible by root.
> @@ -161,3 +170,5 @@ RAM/SWAP in 10240 inodes and it is only accessible by root.
> Hugh Dickins, 4 June 2007
> :Updated:
> KOSAKI Motohiro, 16 Mar 2010
> +Updated:

I bet that should say ":Updated:" now.

> + Chris Down, 13 July 2020
> diff --git a/fs/Kconfig b/fs/Kconfig
> index ff257b81fde5..64d530ba42f6 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -229,6 +229,21 @@ config TMPFS_XATTR
>
> If unsure, say N.
>
> +config TMPFS_INODE64
> + bool "Use 64-bit ino_t by default in tmpfs"
> + depends on TMPFS && 64BIT
> + default n

Okay: I haven't looked up what our last position was on which way the
default should be, but I do recall that we were reluctantly realizing
that we couldn't safely be as radical as we had hoped.

As to "default n" being the default and unnecessary: I don't mind
seeing it there explicitly, let's give the work to whoever wants to
delete that line.

> + help
> + tmpfs has historically used only inode numbers as wide as an unsigned
> + int. In some cases this can cause wraparound, potentially resulting in
> + multiple files with the same inode number on a single device. This option
> + makes tmpfs use the full width of ino_t by default, similarly to the
> + inode64 mount option.

I've just realized that this, and the inode64 Documentation, make no
mention of why you might not want to enable it: it sounds like such a
good thing, with the documentation on why not in include/linux/fs.h.
I'd better add some text to these in the -fix.patch.

> +
> + To override this default, use the inode32 or inode64 mount options.
> +
> + If unsure, say N.
> +
> config HUGETLBFS
> bool "HugeTLB file system support"
> depends on X86 || IA64 || SPARC64 || (S390 && 64BIT) || \
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index eb628696ec66..a5a5d1d4d7b1 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -36,6 +36,7 @@ struct shmem_sb_info {
> unsigned char huge; /* Whether to try for hugepages */
> kuid_t uid; /* Mount uid for root directory */
> kgid_t gid; /* Mount gid for root directory */
> + bool full_inums; /* If i_ino should be uint or ino_t */

Fine, but don't be surprised if I change that eventually,
maybe I'll make it a bit in a bitmask: I've nothing against bools
generally, but like to know the size of my structure fields,
and never sure what size a bool ends up with.

> ino_t next_ino; /* The next per-sb inode number to use */
> ino_t __percpu *ino_batch; /* The next per-cpu inode number to use */
> struct mempolicy *mpol; /* default memory policy for mappings */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 0ae250b4da28..f3126ad7ba3d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -114,11 +114,13 @@ struct shmem_options {
> kuid_t uid;
> kgid_t gid;
> umode_t mode;
> + bool full_inums;
> int huge;
> int seen;
> #define SHMEM_SEEN_BLOCKS 1
> #define SHMEM_SEEN_INODES 2
> #define SHMEM_SEEN_HUGE 4
> +#define SHMEM_SEEN_INUMS 8
> };
>
> #ifdef CONFIG_TMPFS
> @@ -286,12 +288,17 @@ static int shmem_reserve_inode(struct super_block *sb, ino_t *inop)
> ino = sbinfo->next_ino++;
> if (unlikely(is_zero_ino(ino)))
> ino = sbinfo->next_ino++;
> - if (unlikely(ino > UINT_MAX)) {
> + if (unlikely(!sbinfo->full_inums &&
> + ino > UINT_MAX)) {

And don't be surprised if I put that on one line one day :)

> /*
> * Emulate get_next_ino uint wraparound for
> * compatibility
> */
> - ino = 1;
> + if (IS_ENABLED(CONFIG_64BIT))
> + pr_warn("%s: inode number overflow on device %d, consider using inode64 mount option\n",
> + __func__, MINOR(sb->s_dev));

I like that you've added a warning; but it comes too late for a remount,
doesn't it? We could do a countdown - "Only 1 billion more inode numbers
until overflow!" - that would be fun :) But a multiplicity of warnings
would be irritating to some and unnoticed by others. I don't know.

> + sbinfo->next_ino = 1;
> + ino = sbinfo->next_ino++;
> }
> *inop = ino;
> }
> @@ -304,6 +311,10 @@ static int shmem_reserve_inode(struct super_block *sb, ino_t *inop)
> * unknown contexts. As such, use a per-cpu batched allocator
> * which doesn't require the per-sb stat_lock unless we are at
> * the batch boundary.
> + *
> + * We don't need to worry about inode{32,64} since SB_KERNMOUNT
> + * shmem mounts are not exposed to userspace, so we don't need
> + * to worry about things like glibc compatibility.
> */
> ino_t *next_ino;
> next_ino = per_cpu_ptr(sbinfo->ino_batch, get_cpu());
> @@ -3397,6 +3408,8 @@ enum shmem_param {
> Opt_nr_inodes,
> Opt_size,
> Opt_uid,
> + Opt_inode32,
> + Opt_inode64,
> };
>
> static const struct constant_table shmem_param_enums_huge[] = {
> @@ -3416,6 +3429,8 @@ const struct fs_parameter_spec shmem_fs_parameters[] = {
> fsparam_string("nr_inodes", Opt_nr_inodes),
> fsparam_string("size", Opt_size),
> fsparam_u32 ("uid", Opt_uid),
> + fsparam_flag ("inode32", Opt_inode32),
> + fsparam_flag ("inode64", Opt_inode64),
> {}
> };
>
> @@ -3487,6 +3502,18 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
> break;
> }
> goto unsupported_parameter;
> + case Opt_inode32:
> + ctx->full_inums = false;
> + ctx->seen |= SHMEM_SEEN_INUMS;
> + break;
> + case Opt_inode64:
> + if (sizeof(ino_t) < 8) {
> + return invalfc(fc,
> + "Cannot use inode64 with <64bit inums in kernel\n");
> + }
> + ctx->full_inums = true;
> + ctx->seen |= SHMEM_SEEN_INUMS;
> + break;
> }
> return 0;
>
> @@ -3578,8 +3605,16 @@ static int shmem_reconfigure(struct fs_context *fc)
> }
> }
>
> + if ((ctx->seen & SHMEM_SEEN_INUMS) && !ctx->full_inums &&
> + sbinfo->next_ino > UINT_MAX) {
> + err = "Current inum too high to switch to 32-bit inums";
> + goto out;
> + }
> +
> if (ctx->seen & SHMEM_SEEN_HUGE)
> sbinfo->huge = ctx->huge;
> + if (ctx->seen & SHMEM_SEEN_INUMS)
> + sbinfo->full_inums = ctx->full_inums;
> if (ctx->seen & SHMEM_SEEN_BLOCKS)
> sbinfo->max_blocks = ctx->blocks;
> if (ctx->seen & SHMEM_SEEN_INODES) {
> @@ -3619,6 +3654,29 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
> if (!gid_eq(sbinfo->gid, GLOBAL_ROOT_GID))
> seq_printf(seq, ",gid=%u",
> from_kgid_munged(&init_user_ns, sbinfo->gid));
> +
> + /*
> + * Showing inode{64,32} might be useful even if it's the system default,
> + * since then people don't have to resort to checking both here and
> + * /proc/config.gz to confirm 64-bit inums were successfully applied
> + * (which may not even exist if IKCONFIG_PROC isn't enabled).
> + *
> + * We hide it when inode64 isn't the default and we are using 32-bit
> + * inodes, since that probably just means the feature isn't even under
> + * consideration.
> + *
> + * As such:
> + *
> + * +-----------------+-----------------+
> + * | TMPFS_INODE64=y | TMPFS_INODE64=n |
> + * +------------------+-----------------+-----------------+
> + * | full_inums=true | show | show |
> + * | full_inums=false | show | hide |
> + * +------------------+-----------------+-----------------+
> + *
> + */

Thanks for spelling this out: it may be the most contentious part
of the patch. I don't disagree with your decision, but I can imagine
it causing annoyance. Let's start off with it as you decided, but be
ready to be persuaded differently. I have a growing inclination
towards the 64-bit kernel showing ",inode32" but never ",inode64".

> + if (IS_ENABLED(CONFIG_TMPFS_INODE64) || sbinfo->full_inums)
> + seq_printf(seq, ",inode%d", (sbinfo->full_inums ? 64 : 32));
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> /* Rightly or wrongly, show huge mount option unmasked by shmem_huge */
> if (sbinfo->huge)
> @@ -3667,6 +3725,8 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
> ctx->blocks = shmem_default_max_blocks();
> if (!(ctx->seen & SHMEM_SEEN_INODES))
> ctx->inodes = shmem_default_max_inodes();
> + if (!(ctx->seen & SHMEM_SEEN_INUMS))
> + ctx->full_inums = IS_ENABLED(CONFIG_TMPFS_INODE64);
> } else {
> sb->s_flags |= SB_NOUSER;
> }
> @@ -3684,6 +3744,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
> }
> sbinfo->uid = ctx->uid;
> sbinfo->gid = ctx->gid;
> + sbinfo->full_inums = ctx->full_inums;
> sbinfo->mode = ctx->mode;
> sbinfo->huge = ctx->huge;
> sbinfo->mpol = ctx->mpol;
> --
> 2.27.0

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