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

KAMEZAWA Hiroyuki
2011-07-15T00:02:35Z

On Thu, 14 Jul 2011 11:07:00 -0400
Christoph Hellwig <hch@infradead.org> wrote:

> 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.
>

Hmm. I'll prepare 2 functions for memcg
1. asynchronous memory reclaim as kswapd does.
2. dirty_ratio

please remove ->writepage 1st. It may break memcg but it happens sometimes.
We'll do fix.

Thanks,
-Kame