Date: Mon, 2 Mar 2020 01:46:25 +0100 From: Willem Jan Withagen <wjw@digiware.nl> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: What is the exact function of the offset parameter in preadv?? Message-ID: <ecdc6007-e6dc-a30e-97fc-e18634d93430@digiware.nl>
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Hi, Tried looking in the manual page, but it does not really describes what off_t offset does with a preadv... I suspect that is the offset in the file to start reading from? Next question is: What if the value is negative? Are we then reading from the end of the file/device if it is seekable? Manual page says: The pread() and preadv() system calls may also return the following errors: [EINVAL] The offset value was negative. But then I have this behyve code that gets calls like: block_proc_read: buf=0x0, cnt=1, len=512, off=-1024(0xfffffc00). block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00). block_proc_read: buf=0x0, cnt=1, len=512, off=-1024(0xfffffc00). block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00). block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00). block_proc_read: buf=0x0, cnt=1, len=2048, off=-32256(0xffff8200). block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200). block_proc_read: buf=0x0, cnt=1, len=512, off=0(0x0). block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200). block_proc_read: buf=0x0, cnt=1, len=512, off=0(0x0). block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200). block_proc_read: buf=0x0, cnt=1, len=16384, off=1024(0x400). block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00). block_proc_read: buf=0x0, cnt=1, len=16384, off=-16896(0xffffbe00). block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00). Where these are the very first calls that a bhyve loaded image executes. which I guess is for probing the bootdisk to see where/what it needs to mount a root disk. Below bhyve boot output of a not working boot, and a working boot. The read trace above is from the working bhyve boot. So what are the negative offsets? --WjW Compare Using block_rbd_read for reading from a Rados device ============= vtblk0: <VirtIO Block Adapter> on virtio_pci1 vtblk0: 134217727MB (34359738366 4096 byte sectors) ahci0: <Intel ICH8 AHCI SATA controller> mem 0xc0004000-0xc00043ff irq 18 at device 4.0 on pci0 ahci0: AHCI v1.30 with 6 6Gbps ports, Port Multiplier not supported atkbdc0: <Keyboard controller (i8042)> port 0x60,0x64 irq 1 on acpi0 atkbd0: <AT Keyboard> irq 1 on atkbdc0 kbd0 at atkbd0 atkbd0: [GIANT-LOCKED] driver bug: Unable to set devclass (class: atkbdc devname: (unknown)) psm0: <PS/2 Mouse> irq 12 on atkbdc0 psm0: [GIANT-LOCKED] psm0: model Generic PS/2 mouse, device ID 0 uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0 uart0: console (9600,n,8,1) uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0 vga0: <Generic ISA VGA> at port 0x3b0-0x3bb iomem 0xb0000-0xb7fff pnpid PNP0900 on isa0 Timecounters tick every 10.000 msec usb_needs_explore_all: no devclass Trying to mount root from ufs:/dev/ada0p2 [rw]... mountroot: waiting for device /dev/ada0p2... Mounting from ufs:/dev/ada0p2 failed with error 19. Loader variables: vfs.root.mountfrom=ufs:/dev/ada0p2 vfs.root.mountfrom.options=rw Manual root filesystem specification: <fstype>:<device> [options] Mount <device> using filesystem <fstype> and with the specified (optional) option list. eg. ufs:/dev/da0s1a zfs:zroot/ROOT/default cd9660:/dev/cd0 ro (which is equivalent to: mount -t cd9660 -o ro /dev/cd0 /) ? List valid disk boot devices . Yield 1 second (for background tasks) <empty line> Abort manual input ============= With booting from a normal file device: atkbdc0: <Keyboard controller (i8042)> port 0x60,0x64 irq 1 on acpi0 atkbd0: <AT Keyboard> irq 1 on atkbdc0 kbd0 at atkbd0 atkbd0: [GIANT-LOCKED] driver bug: Unable to set devclass (class: atkbdc devname: (unknown)) psm0: <PS/2 Mouse> irq 12 on atkbdc0 psm0: [GIANT-LOCKED] psm0: model Generic PS/2 mouse, device ID 0 uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0 uart0: console (9600,n,8,1) uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0 vga0: <Generic ISA VGA> at port 0x3b0-0x3bb iomem 0xb0000-0xb7fff pnpid PNP0900 on isa0 Timecounters tick every 10.000 msec usb_needs_explore_all: no devclass Trying to mount root from ufs:/dev/ada0p2 [rw]... Root mount waiting for: CAM ada0 at ahcich0 bus 0 scbus0 target 0 lun 0 ada0: <BHYVE SATA DISK 001> ACS-2 ATA SATA 3.x device ada0: Serial Number BHYVE-8C7D-D922-AD47 ada0: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 8192bytes) ada0: Command Queueing enabled ada0: 20480MB (41943040 512 byte sectors) ada1 at ahcich6 bus 0 scbus1 target 0 lun 0 ada1: <BHYVE SATA DISK 001> ACS-2 ATA SATA 3.x device ada1: Serial Number BHYVE-4318-7904-5D51 ada1: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 8192bytes) ada1: Command Queueing enabled ada1: 20480MB (41943040 512 byte sectors) mountroot: waiting for device /dev/ada0p2... WARNING: / was not properly dismounted WARNING: /: mount pending error: blocks 168 files 1 ==========
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