Re99: how to flush out cache.?

[Stephan Uphoff]

Matthew Dillon wrote:
 
>    The idea with the fix is as follows:
> 
>    * It's hard to make the VM system invalidate buffers, so don't try.
>
	
But it is easy to throw away clean buffers since they are nicely sorted
and easy to release.
Efficient locking is a bit tricky but should not be to bad. 

>
>      Instead, make UFS recognize the IO_INVAL ioflag and mark the buffer
>      appropriately in the putpages -> generic put pages -> VOP_WRITE() path.

This is a nice optimization - so we don't have to delete the buffer
once it is clean.

> 
>      This only occurs if the msync'd map was writable.  If the msync'd map was
>      only readable, then the data is not flushed through the vnode system and
>      the pages will not be removed if there happens to be a buffer associated
>      with them, even if the buffer is clean.

I would advise to also flush read only mapped data. (like 5.x)
On writeable maps clean pages will also not be removed if a buffer is
associated with them. 
( For example the sequence  msync(a,l,MS_SYNC); msync(a,l,MS_INVALIDATE|MS_SYNC
)
  will not invalidate/destroy the buffers)

> 
>      I think this is reasonable.  Not perfect, but reasonable.
> 
>    * Set clean_only to TRUE when calling vm_object_page_remove(), which
>      causes vm_object_page_remove() to ignore wired or dirty pages.  This
>      code is called for both readable and writable maps so we can't just 
>      remove the page unconditionallty, hence clean_only should be
>      set to TRUE.

Yes - see earlier email. 

> 
>      This means that any pages associated with the buffer cache will NOT
>      be removed if the map was read-only.  If the map was writable, then
>      the IO_INVAL fix above will destroy the buffer and the page should
>      wind up not being wired any more, and thus will be properly removed.
> 
>     That's it!
> 
> 						-Matt 

The patch solves the data loss problem.
This is the really critical part.

When I get the time (probably not next week) I will write a patch
to release the cached buffers that would prevent page removal.


	Stephan