Messages in this thread |  | | Date | Tue, 8 Jan 2013 20:14:55 -0800 (PST) | Subject | Re: [block] allow blk_flush_policy to return REQ_FSEQ_DATA independent of *FLUSH | From | Ajith Kumar <> |
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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@gmail.com 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 > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/
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