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The read-ahead code from r298230 made it likely the boot code would read

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beyond the end of disk. r298900 added code to prevent this.  Some BIOSes
cause significant delays if asked to read past end-of-disk.

We never trusted the BIOS to accurately report the sectorsize of disks
before and this set of changes.  Unfortuately they interact badly with
the infamous >2TB wraparound bugs.  We have a number of relatively-recent
machines in the FreeBSD.org cluster where the BIOS reports 3TB disks as 1TB.

With pre-r298900 they work just fine.  After r298900 they stop working if
the boot environment attempts to access anything outside the first 1TB on
the disk.  'ZFS: I/O error, all block copies unavailable' etc.  It affects
both UFS and ZFS if they try to boot from large volumes.

This change replaces the blind trust of the BIOS end-of-disk reporting
with a read-ahead clip to prevent reads crossing the of end-of-disk
boundary.  Since 2^32 (2TB) size reporting truncation is not uncommon,
the clipping is done on 2TB aliases of the reported end-of-disk.
ie: a 3TB disk reported as 1TB has readahead clipped at 1TB, 3TB, 5TB, ...
as one of them is likely to be the real end-of-disk.

This should make the loader on these broken machines behave the same as
traditional pre-r298900 loader behavior, without disabling read-ahead.

PR:		212139
Discussed with:	tsoome, allanjude
This commit is contained in:
peter 2016-08-28 20:39:33 +00:00
parent c9c476987e
commit 000d7b586a
1 changed files with 10 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
sys/boot/i386/libi386/biosdisk.c
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@ -497,7 +497,7 @@ bd_realstrategy(void *devdata, int rw, daddr_t dblk, size_t offset, size_t size,
char *buf, size_t *rsize)
{
struct disk_devdesc *dev = (struct disk_devdesc *)devdata;
int blks;
int blks, remaining;
#ifdef BD_SUPPORT_FRAGS /* XXX: sector size */
char fragbuf[BIOSDISK_SECSIZE];
size_t fragsize;
@ -513,14 +513,15 @@ bd_realstrategy(void *devdata, int rw, daddr_t dblk, size_t offset, size_t size,
if (rsize)
*rsize = 0;
if (dblk >= BD(dev).bd_sectors) {
DEBUG("IO past disk end %llu", (unsigned long long)dblk);
return (EIO);
}
if (dblk + blks > BD(dev).bd_sectors) {
/* perform partial read */
blks = BD(dev).bd_sectors - dblk;
/*
* Perform partial read to prevent read-ahead crossing
* the end of disk - or any 32 bit aliases of the end.
* Signed arithmetic is used to handle wrap-around cases
* like we do for TCP sequence numbers.
*/
remaining = (int)(BD(dev).bd_sectors - dblk); /* truncate */
if (remaining > 0 && remaining < blks) {
blks = remaining;
size = blks * BD(dev).bd_sectorsize;
DEBUG("short read %d", blks);
}