I didn't create any mount point.
I did all three partition directly on mmc with gparted then I put the card in the n900.
I've also deleted every partition and then created again (also only the ext4), but without results.
I didn't modify my /etc/fstab, because i didn't consider essential.
Today I realize that if I use the swap on sd, the ext4 sometimes moves on mmc1p1 mount point.
Tomorrow I'll try changing the partition to ext3.