Hi, Recently i've started with development of kernel module, which uses existing block devices/partitions as backend and represents itself as scsi disk. i've found good start point - scsi_debug module which is in official kernel tree under drivers/scsi/ i suppose you are aware about this module. So in fact it is low level scsi adapter which represents itself as scsi disk /dev/sd? on the system and uses RAM as backend storage. actually what i need! so my general idea was to use this it as base and do some changes in IO. scsi_debug IO is trivial. It allocates memory by vmalloc() in __init to fake_storep pointer. and then use it in IO. but the problem appeared for me to rewrite these places of fake_storep usage to write/read to existing block device. i've investigated of how device mapper achieve this. - by generic block layer. the key tool is submit_bio(). Well, submit_bio is not difficult and i was able to read and write bios to/from block device. How writing is done in scsi_debug: scsi_debug has .queuecommand = scsi_debug_queuecommand which switch scsi command to READ/WRITE_6/10/12/16/32 and calls resp_read/write() they both call do_device_access() and it in turn fetch_to_dev_buffer or fill_from_dev_buffer if read or write required. both them use scsi_sg_copy_to_buffer or sg_copy_from_buffer... so these functions eventually fetch/fill scsi response buffer from existing fake RAM storage. all seems to be fine and clear before trying to replace RAM storage by real block device. the problem is that then we are in scsi_debug_queuecommand we can not block(sleep). any attempt to call function that may block causes oops. or even hang. with BUG: sleeping function called from invalid context because of upstream scsi layers holds locks while handling request as for example scsi_dispatch_cmd from drivers/scsi/scsi.c: ... spin_lock_irqsave(host->host_lock, flags); ... in case of scsi WRITE command i have workaround. i can simply allocate some temporary buffer with GFP_ATOMIC to handle write request (its max size 512K - scsi mid layer breaks request to low level adapter in these chunks). pass this buffer to scsi_sg_copy_to_buffer - it will fill my temporary buffer without blocking. then setup workqueue work and actual submit_bio() to block device will be handled later from thread context where such operations are normal. the problem is with response to READ. scsi_debug just uses sg_copy_from_buffer from RAM storage - where data is always uptodate. but in my case i need to read data from block device by submit_bio first and only after this can use buffer updated to pass it to sg_copy_from_buffer... I still don't have idea how to accomplish this. does anyone have any idea? thank you in advance for any suggestions or criticism. thanks, Dmitry