kern/60147: /dev/drum i/o seems racy

["imuwoto%gmail.com@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60147
>Category:       kern
>Synopsis:       /dev/drum i/o seems racy
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Mar 30 22:40:00 +0000 2026
>Originator:     YAMAMOTO Takashi
>Release:        -current
>Organization:
>Environment:
>Description:
since the initial uvm import, uvm_swap.c has had a comment:

        /*
         * convert block number to swapdev.   note that swapdev can't
         * be yanked out from under us because we are holding resources
         * in it (i.e. the blocks we are doing I/O on).
         */

the assumption doesn't seem true for /dev/drum i/o.

>How-To-Repeat:
code inspection
>Fix: