[dm-devel] New dm-bufio with shrinker API

[Christoph Hellwig]

On Tue, Sep 06, 2011 at 11:57:00AM -0400, Mikulas Patocka wrote:
> > IS there any reason you'll need a fixed size?  This is fairly similar in
> > concept to the XFS buffercache, which does perfectly well by allocation
> > memory as needed, and letting the shrinker reclaim buffers when under
> > memory pressure.
> 
> It is possible to make unlimited size. --- the question: is the shrinker 
> run when we exhaust vmalloc arena?
> 
> dm-bufio cache uses vmalloc arena under some circumstances. On some 
> architectures (for example i386), vmalloc arena is smaller than main 
> memory, therefore it may overflow before main memory does.
> 
> What does XFS do when vmalloc arena is exhausted?

At this point shrinkers do not handle vmalloc space, although we could
add them.  In the default configuration XFS uses very little vmalloc
space in the buffer cache - only the 8 log buffers are vmapped, and
those can't be reclaimed anyway.  During log recovery or if using the
non-standard larger directory block mkfs option it can consume a larger
amount of vmalloc space, and we have run into problems because of that,
e.g. take a look at the loop around vm_map_ram() in _xfs_buf_map_pages()
that we had to add as a workaround, and the commit introducing it for
more details (a19fb380).

Just curious, why do you need the buffers to be vmapped?  If we'd design
the dir2 format these days we'd make sure it is aligned in a way that
we could deal with individually mapped pages.