Hi All,I have posted this a while ago on strange behavior I see when "dd" opens and reads the disk presented by my virtual HBA. I am attaching the code in this email.
Basically, I have this virtual HBA which is being served by a user space SCSI engine. It basically transports SCSI commands out side kernel and get serviced by a user space scsi engine.
I have successfully implemented this using read and write system call to a character device. So the SCSI request buffer is copied back and forth user and kernel space for every request. It works alright.
Now i wanted a method of sharing SCSI request_buffer between user space and kernel without doing copy_to_user and copy_from_user of the request_buffer. I have implemented memmap and facing some challenges. I am memory mapping the scsi request_buffer to the user space. But i am facing a problem with page cache.
The SCSI device presented by my HBA is /dev/sdb (for eg:). If I open this device and read a block ( 512 bytes). I can see a SCSI read coming down my HBA. But I also a see a SCSI write later on when I close /dev/sdb. Up on investigation I found that it is the effect of page cache. When /dev/sdb opened and initiated a read, a page cache is already allocated. When I do memmap, I am actually mem mapping that page buffer to my user space SCSI engine. When the SCSI engine service the SCSI read, it writes the SCSI READ output to the buffer making it dirty. When the application closed /dev/sdb, the kernel causes all the dirty pages to be written and causing the SCSI write which is what I am seeing. This means that for every SCSI read, i will see a SCSI write.It is a very strange implementation from my side. Even Xen source is doing the same memory mapping technique and I do not know how it avoid this page cache interference.
Can some one throw some lights on this for me to prevent this page cache interference? I have done bit of research on marking the pages/buffer clean after writing to the memory mapped area. But I could not find anything conclusive.
Thanks in advance, Aboo Aboo Valappil wrote:
Hi All, Thanks for all your support for my previous posts. I have two questions.1. Is it safe to assume that all the SG elements of request_buffer are page aligned? (Except the first one and last one), I mean, it is always multiple of pages?2. What are situations reading a scsi disk (using dd ) cause a write to the disk? When I read my virtual scsi disk using dd command, block layer is generating lot of SCSI write, as well as reads. Any thoughts? Besides this observation, everything is working fine.Thanks in advance. Aboo - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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