Re: [PATCH v3 2/2] md: simplify md_seq_ops

[Yu Kuai]

Hi,

在 2024/01/09 9:21, Yu Kuai 写道:
> Hi,
>
> 在 2024/01/09 7:38, Song Liu 写道:
> > On Tue, Sep 26, 2023 at 11:19 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > Before this patch, the implementation is hacky and hard to understand:
> > >
> > > 1) md_seq_start set pos to 1;
> > > 2) md_seq_show found pos is 1, then print Personalities;
> > > 3) md_seq_next found pos is 1, then it update pos to the first mddev;
> > > 4) md_seq_show found pos is not 1 or 2, show mddev;
> > > 5) md_seq_next found pos is not 1 or 2, update pos to next mddev;
> > > 6) loop 4-5 until the last mddev, then md_seq_next update pos to 2;
> > > 7) md_seq_show found pos is 2, then print unused devices;
> > > 8) md_seq_next found pos is 2, stop;
> > >
> > > This patch remove the magic value and use seq_list_start/next/stop()
> > > directly, and move printing "Personalities" to md_seq_start(),
> > > "unsed devices" to md_seq_stop():
> > >
> > > 1) md_seq_start print Personalities, and then set pos to first mddev;
> > > 2) md_seq_show show mddev;
> > > 3) md_seq_next update pos to next mddev;
> > > 4) loop 2-3 until the last mddev;
> > > 5) md_seq_stop print unsed devices;
> > >
> > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Just realized this introduced a behavior change:
> >
> > When there is not md devices, before this patch, we have
> >
> > [root@eth50-1 ~]# cat /proc/mdstat
> > Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
> > unused devices: <none>
> >
> > After this patch, "cat /proc/mdstat" returns nothing. This causes
> > some confusion for users who want to read "Personalities" line,
> > for example, the mdadm test suite reads it.
> >
> > I haven't figured out the best fix yet.
>
> Yes, that's a problem. And after reviewing seq_read_iter() in detail, I
> realize that I also can't use seq_printf() in m->op->start() directly,
> because if seq buffer overflowed, md_seq_start() can be called more than
> once.
>
> I'll fix these problems soon.
How about following patch(already tested)?

Thanks,
Kuai

diff --git a/drivers/md/md.c b/drivers/md/md.c
index e351e6c51cc7..289d3d89e73d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8135,6 +8135,19 @@ static void status_unused(struct seq_file *seq)
        seq_printf(seq, "\n");
 }

+static void status_personalities(struct seq_file *seq)
+{
+       struct md_personality *pers;
+
+       seq_puts(seq, "Personalities : ");
+       spin_lock(&pers_lock);
+       list_for_each_entry(pers, &pers_list, list)
+               seq_printf(seq, "[%s] ", pers->name);
+
+       spin_unlock(&pers_lock);
+       seq_puts(seq, "\n");
+}
+
 static int status_resync(struct seq_file *seq, struct mddev *mddev)
 {
        sector_t max_sectors, resync, res;
@@ -8273,43 +8286,53 @@ static int status_resync(struct seq_file *seq, 
struct mddev *mddev)
        return 1;
 }

+#define MDDEV_NONE (void *)1
+
 static void *md_seq_start(struct seq_file *seq, loff_t *pos)
        __acquires(&all_mddevs_lock)
 {
-       struct md_personality *pers;
-
-       seq_puts(seq, "Personalities : ");
-       spin_lock(&pers_lock);
-       list_for_each_entry(pers, &pers_list, list)
-               seq_printf(seq, "[%s] ", pers->name);
-
-       spin_unlock(&pers_lock);
-       seq_puts(seq, "\n");
        seq->poll_event = atomic_read(&md_event_count);
-
        spin_lock(&all_mddevs_lock);

-       return seq_list_start(&all_mddevs, *pos);
+       if (!list_empty(&all_mddevs))
+               return seq_list_start(&all_mddevs, *pos);
+       else if (*pos == 0)
+               return MDDEV_NONE;
+       else
+               return NULL;
 }

 static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
+       if (v == MDDEV_NONE) {
+               ++*pos;
+               return NULL;
+       }
+
        return seq_list_next(v, &all_mddevs, pos);
 }

 static void md_seq_stop(struct seq_file *seq, void *v)
        __releases(&all_mddevs_lock)
 {
-       status_unused(seq);
        spin_unlock(&all_mddevs_lock);
 }
 static int md_seq_show(struct seq_file *seq, void *v)
 {
-       struct mddev *mddev = list_entry(v, struct mddev, all_mddevs);
+       struct mddev *mddev;
        sector_t sectors;
        struct md_rdev *rdev;

+       if (v == MDDEV_NONE) {
+               status_personalities(seq);
+               status_unused(seq);
+               return 0;
+       }
+
+       mddev = list_entry(v, struct mddev, all_mddevs);
+       if (mddev == list_first_entry(&all_mddevs, struct mddev, 
all_mddevs))
+               status_personalities(seq);
        if (!mddev_get(mddev))
                return 0;

@@ -8385,6 +8408,10 @@ static int md_seq_show(struct seq_file *seq, void *v)
        }
        spin_unlock(&mddev->lock);
        spin_lock(&all_mddevs_lock);
+
+       if (mddev == list_last_entry(&all_mddevs, struct mddev, all_mddevs))
+               status_unused(seq);
+
        if (atomic_dec_and_test(&mddev->active))
                __mddev_put(mddev);




> Thanks,
> Kuai
>
> > Thanks,
> > Song
> >
> > .
>
> .