K 10
svn:author
V 3
cem
K 8
svn:date
V 27
2019-09-11T21:24:14.755445Z
K 7
svn:log
V 683
buf: Add B_INVALONERR flag to discard data

Setting the B_INVALONERR flag before a synchronous write causes the buf
cache to forcibly invalidate contents if the write fails (BIO_ERROR).

This is intended to be used to allow layers above the buffer cache to make
more informed decisions about when discarding dirty buffers without
successful write is acceptable.

As a proof of concept, use in msdosfs to handle failures to mark the on-disk
'dirty' bit during rw mount or ro->rw update.

Extending this to other filesystems is left as future work.

PR:		210316
Reviewed by:	kib (with objections)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D21539

END