svn commit: r246293 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs

[Andriy Gapon]

on 03/02/2013 21:23 Andriy Gapon said the following:
> on 03/02/2013 20:42 Andriy Gapon said the following:
>> -			vm_page_busy(pp);
>> -			vm_page_undirty(pp);
> 
> ZFS lookup_page previously had two interesting bugs that almost canceled each other.
> 
> The first bug was that vm_page_undirty was called even if we read from a
> ZFS-backed page into a user buffer.  Obviously this action didn't actually
> magically make the page clean.
> The second bug was that vm_page_undirty was called without doing
> pmap_remove_write() first and so, if the page was actually modified, the
> modified bit leaked back from a pmap to the dirty bits.  So no harm was done.
> 
> On the other hand, if a ZFS-backed dirty page was over-written with a content
> from ARC (which was first put there from a userland buffer), then the page
> stayed marked dirty for all the same reason (lack of pmap_remove_write).  And so
> exactly the same data would have to be re-written to ARC / disk again.

Eh, just realized that on the third hand, this cancelled yet another bug :-)
vm_page_undirty was called on the whole page even when only a part of it
overlapped with the buffer and thus was supposed to become clean.


-- 
Andriy Gapon