Re: kern/54470: Adapt FFS to work with HP-UX UFS1 variant

[mlelstv%serpens.de@localhost (Michael van Elst)]

The following reply was made to PR kern/54470; it has been noted by GNATS.

From: mlelstv%serpens.de@localhost (Michael van Elst)
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/54470: Adapt FFS to work with HP-UX UFS1 variant
Date: Sat, 17 Aug 2019 11:15:28 -0000 (UTC)

 perseant%hhhh.org@localhost (Konrad Schroder) writes:
 
 >Can you be more specific?  It gives the right values for my HP-UX image, 
 >and for existing NetBSD file systems.
 
 >>   >+#define BTODB(fs, size) ((size) >> ((fs)->fs_fshift - (fs)->fs_fsbtodb))
 >>   >+#define DBTOB(fs, bc)   ((bc) << ((fs)->fs_fshift - (fs)->fs_fsbtodb))
 
 The original BSD code used definitions like these above as a 'disk offset'
 referred to physical block numbers. This is still used that way in
 userland.
 
 NetBSD disk drivers always use logical block numbers for a block size
 of 512 bytes (DEV_BSIZE) and filesystems ignore the superblock values when
 translating between filesystem block numbers and disk block numbers.
 
 For FFS the macros FFS_FSBTODB / FFS_DBTOFSB are used to do this conversion.
 They are differently defined for kernel and userland to address the different
 views.
 
 btodb / dbtob are originally MD macros (then unified) that always used
 a platform specific unit, i.e. DEV_BSIZE, to specify disk offsets. This
 unit is independent of a particular filesystem and the physical blocks of
 a specific device.
 
 Now what unit does di_blocks use, it's only used for reporting and quota
 calculations ? The original code used btodb(file_length), i.e. the values
 used the platform-wide DEV_BSIZE as a unit and that's what we still use
 today.
 
 Your patch changes that and di_blocks is now using the physical block
 size of the particular disk (recorded in the superblock) as the unit.
 Apparently that's what HP-UX is doing.
 
 I see two issues: you added another view on disk offsets and you break
 existing filesystems that use a physical sector size != 512.
 
 Apparently we need conditional code to handle di_blocks for the
 different systems.
 
 
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                                 Michael van Elst
 Internet: mlelstv%serpens.de@localhost
                                 "A potential Snark may lurk in every tree."