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SubjectRe: [PATCH V5 0/9] Enable ext4 support for per-file/directory DAX operations
Ted,

Sorry for the top post but did you catch this reply? Generally the patch looks
good but I had a couple of questions because I don't fully grok the mount code
especially with regard to EXT2 support.

If you already saw it sorry for bothering you I just know that our email
servers sometimes 'file' things for me and I miss them... ;-)

Thanks,
Ira

On Fri, May 29, 2020 at 11:12:38AM -0700, 'Ira Weiny' wrote:
> On Fri, May 29, 2020 at 12:17:17AM -0400, Theodore Y. Ts'o wrote:
> > On Thu, May 28, 2020 at 10:54:41PM -0400, Theodore Y. Ts'o wrote:
> > >
> > > Thanks, applied to the ext4-dax branch.
> > >
> >
> > I spoke too soon. While I tried merging with the ext4.git dev branch,
> > a merge conflict made me look closer and I realize I needed to make
> > the following changes (see diff between your patch set and what is
> > currently in ext4-dax).
> >
> > Essentially, I needed to rework the branch to take into account commit
> > e0198aff3ae3 ("ext4: reject mount options not supported when
> > remounting in handle_mount_opt()").
> >
> > The problem is that if you allow handle_mount_opt() to apply the
> > changes to the dax settings, and then later on, ext4_remount() realize
> > that we're remounting, and we need to reject the change, there's a
> > race if we restore the mount options to the original configuration.
> > Specifically, as Syzkaller pointed out, between when we change the dax
> > settings and then reset them, it's possible for some file to be opened
> > with "wrong" dax setting, and then when they are reset, *boom*.
> >
> > The correct way to deal with this is to reject the mount option change
> > much earlier, in handle_mount_opt(), *before* we mess with the dax
> > settings.
> >
> > Please take a look at the ext4-dax for the actual changes which I
> > made.
> >
> > Cheers,
> >
> > - Ted
> >
> >
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 3658e3016999..9a37d70394b2 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -1733,7 +1733,7 @@ static int clear_qf_name(struct super_block *sb, int qtype)
> > #define MOPT_NO_EXT3 0x0200
> > #define MOPT_EXT4_ONLY (MOPT_NO_EXT2 | MOPT_NO_EXT3)
> > #define MOPT_STRING 0x0400
> > -#define MOPT_SKIP 0x0800
>
> I think we still need MOPT_SKIP...
>
> This was put in to skip these options when printing to deal with printing only
> dax=inode when it was specified by the user.
>
> Ah but I see now. By taking MOPT_SET away you have created the same behavior?
>
> This is orthogonal to the remount issue right?
>
> > +#define MOPT_NO_REMOUNT 0x0800
> >
> > static const struct mount_opts {
> > int token;
> > @@ -1783,18 +1783,15 @@ static const struct mount_opts {
> > {Opt_min_batch_time, 0, MOPT_GTE0},
> > {Opt_inode_readahead_blks, 0, MOPT_GTE0},
> > {Opt_init_itable, 0, MOPT_GTE0},
> > - {Opt_dax, EXT4_MOUNT_DAX_ALWAYS, MOPT_SET | MOPT_SKIP},
> > - {Opt_dax_always, EXT4_MOUNT_DAX_ALWAYS,
> > - MOPT_EXT4_ONLY | MOPT_SET | MOPT_SKIP},
> > - {Opt_dax_inode, EXT4_MOUNT2_DAX_INODE,
> > - MOPT_EXT4_ONLY | MOPT_SET | MOPT_SKIP},
> > - {Opt_dax_never, EXT4_MOUNT2_DAX_NEVER,
> > - MOPT_EXT4_ONLY | MOPT_SET | MOPT_SKIP},
> > + {Opt_dax, 0, MOPT_NO_REMOUNT},
> > + {Opt_dax_always, 0, MOPT_NO_REMOUNT},
> > + {Opt_dax_inode, 0, MOPT_NO_REMOUNT},
> > + {Opt_dax_never, 0, MOPT_NO_REMOUNT},
>
> Even if MOPT_SET is redundant. Why don't we need still need MOPT_EXT4_ONLY?
>
> And why don't we need to associate the defines; EXT4_MOUNT_DAX_ALWAYS etc?
>
> > {Opt_stripe, 0, MOPT_GTE0},
> > {Opt_resuid, 0, MOPT_GTE0},
> > {Opt_resgid, 0, MOPT_GTE0},
> > - {Opt_journal_dev, 0, MOPT_NO_EXT2 | MOPT_GTE0},
> > - {Opt_journal_path, 0, MOPT_NO_EXT2 | MOPT_STRING},
> > + {Opt_journal_dev, 0, MOPT_NO_EXT2 | MOPT_GTE0 | MOPT_NO_REMOUNT},
> > + {Opt_journal_path, 0, MOPT_NO_EXT2 | MOPT_STRING | MOPT_NO_REMOUNT},
> > {Opt_journal_ioprio, 0, MOPT_NO_EXT2 | MOPT_GTE0},
> > {Opt_data_journal, EXT4_MOUNT_JOURNAL_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
> > {Opt_data_ordered, EXT4_MOUNT_ORDERED_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
> > @@ -1831,7 +1828,7 @@ static const struct mount_opts {
> > {Opt_jqfmt_vfsv1, QFMT_VFS_V1, MOPT_QFMT},
> > {Opt_max_dir_size_kb, 0, MOPT_GTE0},
> > {Opt_test_dummy_encryption, 0, MOPT_GTE0},
> > - {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
> > + {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET | MOPT_NO_REMOUNT},
> > {Opt_err, 0, 0}
> > };
> >
> > @@ -1929,6 +1926,12 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
> > "Mount option \"%s\" incompatible with ext3", opt);
> > return -1;
> > }
> > + if ((m->flags & MOPT_NO_REMOUNT) && is_remount) {
> > + ext4_msg(sb, KERN_ERR,
> > + "Mount option \"%s\" not supported when remounting",
> > + opt);
> > + return -1;
> > + }
>
> I think this is cleaner!
>
> Thanks, I did test this but not while trying to manipulate files as the same time
> as a remount. So a race would not have been caught.
>
> Thanks!
> Ira
>
> >
> > if (args->from && !(m->flags & MOPT_STRING) && match_int(args, &arg))
> > return -1;
> > @@ -2008,11 +2011,6 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
> > }
> > sbi->s_resgid = gid;
> > } else if (token == Opt_journal_dev) {
> > - if (is_remount) {
> > - ext4_msg(sb, KERN_ERR,
> > - "Cannot specify journal on remount");
> > - return -1;
> > - }
> > *journal_devnum = arg;
> > } else if (token == Opt_journal_path) {
> > char *journal_path;
> > @@ -2020,11 +2018,6 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
> > struct path path;
> > int error;
> >
> > - if (is_remount) {
> > - ext4_msg(sb, KERN_ERR,
> > - "Cannot specify journal on remount");
> > - return -1;
> > - }
> > journal_path = match_strdup(&args[0]);
> > if (!journal_path) {
> > ext4_msg(sb, KERN_ERR, "error: could not dup "
> > @@ -2287,7 +2280,7 @@ static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
> > for (m = ext4_mount_opts; m->token != Opt_err; m++) {
> > int want_set = m->flags & MOPT_SET;
> > if (((m->flags & (MOPT_SET|MOPT_CLEAR)) == 0) ||
> > - (m->flags & MOPT_CLEAR_ERR) || m->flags & MOPT_SKIP)
> > + (m->flags & MOPT_CLEAR_ERR))
> > continue;
> > if (!nodefs && !(m->mount_opt & (sbi->s_mount_opt ^ def_mount_opt)))
> > continue; /* skip if same as the default */
> > @@ -5474,24 +5467,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
> > }
> > }
> >
> > - if ((sbi->s_mount_opt ^ old_opts.s_mount_opt) & EXT4_MOUNT_NO_MBCACHE) {
> > - ext4_msg(sb, KERN_ERR, "can't enable nombcache during remount");
> > - err = -EINVAL;
> > - goto restore_opts;
> > - }
> > -
> > - if ((sbi->s_mount_opt ^ old_opts.s_mount_opt) & EXT4_MOUNT_DAX_ALWAYS ||
> > - (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_NEVER ||
> > - (sbi->s_mount_opt2 ^ old_opts.s_mount_opt2) & EXT4_MOUNT2_DAX_INODE) {
> > - ext4_msg(sb, KERN_WARNING, "warning: refusing change of "
> > - "dax mount option with busy inodes while remounting");
> > - sbi->s_mount_opt &= ~EXT4_MOUNT_DAX_ALWAYS;
> > - sbi->s_mount_opt |= old_opts.s_mount_opt & EXT4_MOUNT_DAX_ALWAYS;
> > - sbi->s_mount_opt2 &= ~(EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);
> > - sbi->s_mount_opt2 |= old_opts.s_mount_opt2 &
> > - (EXT4_MOUNT2_DAX_NEVER | EXT4_MOUNT2_DAX_INODE);
> > - }
> > -
> > if (sbi->s_mount_flags & EXT4_MF_FS_ABORTED)
> > ext4_abort(sb, EXT4_ERR_ESHUTDOWN, "Abort forced by user");
> >

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