After upgrading 2 machines from 9.3 to 10.3-STABLE, on one of them the
l2arc stays empty (capacity alloc = 0), although it is online and gets
accessed. It did work well on 9.3.

I did the following tests:
 * Create a zpool on a stick, with two volumes: one filesystem and one
   cache. The cache stays with alloc=0.
   Export it and move it into the other machine. The cache immediately
   Move it back, the cache stays with alloc=0.
   -> this rules out all zpool/zfs get/set options, as they should
      walk with the pool.
 * Boot the GENERIC kernel. l2arc stays with alloc=0.
   -> this rules out all my nonstandard kernel options.
 * Boot in single user mode. l2arc stays with alloc=0.
   -> this rules out all /etc/* config files.
 * Delete the zpool.cache and reimport pools. l2arc stays with alloc=0.
 * Copy the /boot/loader.conf settings to the other machine. The l2arc
   still works there.

I could not think of any remaining place where this could come from,
except the kernel code itself.
From there, I found these counters nicely incrementing each second:
  kstat.zfs.misc.arcstats.l2_write_buffer_list_iter: 50758
  kstat.zfs.misc.arcstats.l2_write_buffer_list_null_iter: 27121
  kstat.zfs.misc.arcstats.l2_write_buffer_bytes_scanned: 40589375488
But also this counter incrementing:
  kstat.zfs.misc.arcstats.l2_write_full: 14604

Then with some printf in the code I saw these values provided:
        buf_sz = hdr->b_size;
        align = (size_t)1 << dev->l2ad_vdev->vdev_ashift;
        buf_a_sz = P2ROUNDUP(buf_sz, align);
        if ((write_asize + buf_a_sz) > target_sz) {
           full = B_TRUE;

buf_sz =        1536
align =         512
buf_a_sz =      18446744069414585856
write_asize =   0
target_sz =     16777216

where buf_a_sz is obviousely off by (2^64 - 2^32).

Maybe this is an effect of crosscompiling i386 on amd64. But anyway, as long as i386 is still supported, it should not happen.

Now, my real concern is: if this really obvious ... made it undetected until 10.3, how many other missing typecasts are still in the code??

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