Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim

Christoph Hellwig
2011-07-14T15:07:11Z

On Thu, Jul 14, 2011 at 01:46:34PM +0900, KAMEZAWA Hiroyuki wrote:
> > XFS and btrfs already disable writeback from memcg context, as does ext4
> > for the typical non-overwrite workloads, and none has fallen apart.
> >
> > In fact there's no way we can enable them as the memcg calling contexts
> > tend to have massive stack usage.
> >
>
> Hmm, XFS/btrfs adds pages to radix-tree in deep stack ?

We're using a fairly deep stack in normal buffered read/write,
wich is almost 100% common code. It's not just the long callchain
(see below), but also that we put the unneeded kiocb and a vector
of I/O vects on the stack:

vfs_writev
do_readv_writev
do_sync_write
generic_file_aio_write
__generic_file_aio_write
generic_file_buffered_write
generic_perform_write
block_write_begin
grab_cache_page_write_begin
add_to_page_cache_lru
add_to_page_cache
add_to_page_cache_locked
mem_cgroup_cache_charge

this might additionally come from in-kernel callers like nfsd,
which has even more stack space used. And at this point we only
enter the memcg/reclaim code, which last time I had a stack trace
ate up another about 3k of stack space.