On Fri, Mar 22, 2019 at 03:48:44PM +0800, Coly Li wrote:
> In 'commit 752f66a75aba ("bcache: use REQ_PRIO to indicate bio for
> metadata")' REQ_META is replaced by REQ_PRIO to indicate metadata bio.
> This assumption is not always correct, e.g. XFS uses REQ_META to mark
> metadata bio other than REQ_PRIO. This is why Nix noticed that bcache
> does not cache metadata for XFS after the above commit.
>
> Thanks to Dave Chinner, he explains the difference between REQ_META and
> REQ_PRIO from view of file system developer. Here I quote part of his
> explanation from mailing list,
> REQ_META is used for metadata. REQ_PRIO is used to communicate to
> the lower layers that the submitter considers this IO to be more
> important that non REQ_PRIO IO and so dispatch should be expedited.
>
> IOWs, if the filesystem considers metadata IO to be more important
> that user data IO, then it will use REQ_PRIO | REQ_META rather than
> just REQ_META.
>
> Then it seems bios with REQ_META or REQ_PRIO should both be cached for
> performance optimation, because they are all probably low I/O latency
> demand by upper layer (e.g. file system).
>
> So in this patch, when we want to decide whether to bypass the cache,
> REQ_META and REQ_PRIO are both checked. Then both metadata and
> high priority I/O requests will be handled properly.
>
> (Coly Li: backport the original patch for Linux-stable v4.19)
Now queued up, thanks.
greg k-h
