On 2019/2/7 10:26 上午, Dave Chinner wrote: > On Thu, Feb 07, 2019 at 01:24:25AM +0100, Andre Noll wrote: >> On Thu, Feb 07, 10:43, Dave Chinner wrote >>> File data readahead: REQ_RAHEAD >>> Metadata readahead: REQ_META | REQ_RAHEAD >>> >>> drivers/md/bcache/request.c::check_should_bypass(): >>> >>> /* >>> * Flag for bypass if the IO is for read-ahead or background, >>> * unless the read-ahead request is for metadata (eg, for gfs2). >>> */ >>> if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) && >>> !(bio->bi_opf & REQ_PRIO)) >>> goto skip; >>> >>> bcache needs fixing - it thinks REQ_PRIO means metadata IO. That's >>> wrong - REQ_META means it's metadata IO, and so this is a bcache >>> bug. >> >> Do you think 752f66a75abad is bad (ha!) and should be reverted? > > Yes, that change is just broken. From include/linux/blk_types.h: > > __REQ_META, /* metadata io request */ > __REQ_PRIO, /* boost priority in cfq */ > > Hi Dave, > i.e. REQ_META means that it is a metadata request, REQ_PRIO means it > is a "high priority" request. Two completely different things, often > combined, but not interchangeable. I found in file system metadata IO, most of time REQ_META and REQ_PRIO are tagged together for bio, but XFS seems not use REQ_PRIO. Is there any basic principle for when should these tags to be used or not ? e.g. If REQ_META is enough for meta data I/O, why REQ_PRIO is used too. And if REQ_PRIO is necessary, why it is not used in fs/xfs/ code ? > > So, yeah, that needs to be reverted if you want bcache to function > properly for metadata caching. Sure, I will fix this, once I make it clear to me. Thanks for the hint. -- Coly Li
