Re: [PATCH] md/raid1: create serial pool adding rdev to array with serialize_policy=1

On Sun, 2026-07-26 at 14:13 +0200, Mykola Marzhan wrote:
> This reply was drafted with AI assistance.
> 
> > But when a new rdev is added to an existing array in
> > bind_rdev_to_array(),
> > the condition at mddev_create_serial_pool() causes creation of
> > rdev->serial
> > to be skipped. Fix it.
> 
> I ran into the same early return from the other direction, so I can
> confirm the analysis: the new rdev fails rdev_need_serial() (it is
> not
> a write-mostly member with write-behind configured), CollisionCheck
> was never set on it, and mddev_create_serial_pool() returns without
> allocating rdev->serial. The first write routed to it then crashes in
> wait_for_serialization(), which is gated on the array-wide flag and
> assumes the tree exists. serialize_policy is array-wide, so creating
> the pool for every rdev bound while it is set is the right fix.
> 
> Reviewed-by: Mykola Marzhan <[email protected]>
> 
> One interaction to flag. Patch 3 of my P2PDMA series changes that
> raid1 gate from MD_SERIALIZE_POLICY to per-rdev CollisionCheck, so
> that writes which skip write-behind also order against in-flight
> behind I/O on write-mostly members (the behind path already checks
> CollisionCheck; the non-behind path does not):
> 
>  
> https://lore.kernel.org/linux-raid/[email protected]/
> 
> For the rdev in your report the two gates disagree: without your fix
> it has no serial tree, and the CollisionCheck gate would skip
> serialization instead of crashing, hiding the oops but not the bug.
> So your patch is needed either way and should go in first; with it,
> every rdev in a serialize_policy array owns a serial tree, and the
> CollisionCheck gate covers everything the old one did. Patch 3
> carries a Link: to your fix.
> 
> If you see a hole in that reasoning I would rather hear it now than
> after either patch lands.

I fully agree. My patch is just a small bug fix. Your changes go much
deeper.

Regards,
Martin

-- 
Dr. Martin Wilck <[email protected]>
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