update BDI {io,ra}_pages values based on the RT device limits v2

Christoph Hellwig <[email protected]> Mon, 20 Jul 2026 16:08:46 +0200

Newsgroups	org.kernel.vger.linux-xfs,org.kernel.vger.linux-fsdevel
Message-ID	<[email protected]>

Hi all,

we ran into a bit of a funny case where adding an SSD to store metadata
to a (zoned) XFS file system reduced the performance vs using a HDD for
both data and metadata.  It turns out this is due to readahead code
looking at the BDI limits, including the io_pages value that can't be
inspected or tuned from userspace.

Changes since v1:
 - restore the old value at unmount, unless the BDI values changed
   since mount