Write protection on the SSD blocks during the READ

[kaigai]

Right now, we have no write protection mechanism on SSD blocks during the execution of SSD-to-GPU Direct DMA.
To avoid transferring corrupted blocks to GPU, we need to have a mechanism to prevent write operation to the blocks we try to read asynchronously, until end of DMA.
One potential solution is POSIX flock (mandatory mode). However, it seems to me a bit larger than necessity because it also prevents buffered i/o also. All we want to keep is the contents of the block during the DMA execution.

It is sufficient to prevent write to block on more lower layer.