Re: RFC bio flags and ops in v4.10+

[Bart Van Assche <bvanassche@acm.org>]

On 1/21/19 8:45 PM, Ashlie Martinez wrote:
> I was working on porting parts of a file system crash consistency
> checking tool called CrashMonkey [1] to linux kernels 4.9 and 4.14
> when I noticed an inconsistency in how the bio->bi_opf field is
> treated. According to the comments in /include/linux/blk_types.h, the
> REQ_OP should be the upper bits in bi_opf, while the request flags
> should be in the lower bits. In kernel 4.9, the accessor and setter
> methods for this field appear to be correct. However, kernels 4.10+ do
> not properly shift the REQ_OP argument up for either setting it or
> getting it from bi_opf. Is this the intended behavior or is it a
> mistake in how the code was written? Please note that I have not
> checked all the releases between 4.9 and 5.0-rc3, but I know 4.10,
> 4.14, 4.15, and 5.0-rc3 contain the same or similar code for setting
> bio->bi_opf.

Hi Ashlie,

Are you perhaps referring to commit ef295ecf090d ("block: better op and 
flags encoding")? Have you noticed that that commit modified the 
encoding such that the operation ends up in the lower 8 bits and the 
flags in the upper 24 bits? That patch namely includes the following change:

+enum req_flag_bits {
+	__REQ_FAILFAST_DEV =	/* no driver retries of device errors */
+		REQ_OP_BITS,

Bart.