re: bin/57921: swapon: Do not overwrite Linux swap header

[matthew green <mrg%eterna23.net@localhost>]

The following reply was made to PR bin/57921; it has been noted by GNATS.

From: matthew green <mrg%eterna23.net@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: gnats-admin%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost
Subject: re: bin/57921: swapon: Do not overwrite Linux swap header
Date: Sun, 11 Feb 2024 07:36:47 +1100

 i'm confused.  we skip the first PAGE_SIZE bytes on block devices,
 ie, any mbr or gpt accessed for this would be a block device.
 
 so netbsd shouldn't overwrite the first 4096 bytes if it sees
 this as a partition, not a file.  *files* are used from the first
 moment.  see uvm_swap.c:
 
          * for block special files, we want to make sure that leave
          * the disklabel and bootblocks alone, so we arrange to skip
          * over them (arbitrarily choosing to skip PAGE_SIZE bytes).
          * note that because of this the "size" can be less than the
          * actual number of blocks on the device.
          */
         if (vp->v_type == VBLK) {
                 /* we use pages 1 to (size - 1) [inclusive] */
                 size = npages - 1;
                 addr = 1;
         } else {
                 /* we use pages 0 to (size - 1) [inclusive] */
                 size = npages;
                 addr = 0;
 
 if that's failing some how, we should fix that problem.
 
 i'm OK with better support for linux swap, but it shouldn't need to
 create a random wedge to do this.  it should be just workable easily.
 
 
 .mrg.