Re: [block] allow blk_flush_policy to return REQ_FSEQ_DATAindependent of *FLUSH

[Ajith Kumar]

Hello,
Thanks for the response.
A block device driver during initialization would decide if it is capable of supporting FLUSH/FUA or not.  Suppose driver claims FLUSH/FUA support then any bio targeted at this driver with FLUSH bit set(which is commonly set by file system like XFS for doing internal logging) has a data corruption vulnerability in case of an abrupt shutdown.  So, IMO the vulnerability is not open to rare window where driver changes q->flush_flags while IO is in flight, but for a much larger window from time driver comes up and throughout it's life.

Thanks,
Ajith

On Wednesday, 9 January 2013 00:15:31 UTC+5:30, Tejun Heo  wrote:
> Hello,
> 
> 
> 
> On Tue, Jan 08, 2013 at 10:04:23AM -0800, ajithb.kumar@xxxxxxxxx wrote:
> 
> > Hi,
> 
> > Could you please provide clarity on the following.
> 
> > ">   Hmmm... yes, this can become a correctness issue if (and only if)
> 
> > >   blk_queue_flush() is called to change q->flush_flags while requests
> 
> > >   are in-flight;"
> 
> >
> 
> > Could you please clarify as to why is it a correctness issue only if
> 
> > blk_queue_flush() is used to change flush_flags when requests are in
> 
> > flight ?  As I understand, XFS does set WRITE_FLUSH_FUA flag in
> 
> > _xfs_buf_ioapply() function irrespective of whether the underlying
> 
> > device supports flush capabilities or not which will flow into
> 
> > blk_insert_flush().  Is my reading of the code correct and is there
> 
> > a general correctness issue here which potentially results in XFS
> 
> > file system corruption in case of an abrupt shutdown independent of
> 
> > q->flush_flags getting changed while request is in flight.
> 
> 
> 
> My memory is kinda fuzzy at this point but if a queue doesn't support
> 
> flush, its flush_flags should be zero and
> 
> generic_make_request_checks() will clear REQ_FLUSH|REQ_FUA from
> 
> bio->bi_rw so we never hit blk_insert_flush() and the request will be
> 
> processed as a normal IO one; however, if REQ_FLUSH goes off after a
> 
> request passed generic_make_request_checks() but before
> 
> blk_flush_policy(), it'll become null op and its data payload won't
> 
> get written out to the underlying device, which is data corruption.
> 
> 
> 
> Thanks.
> 
> 
> 
> -- 
> 
> tejun
> 
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