Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a_ops->writepages() where possible

Chris Mason
2010-06-15T19:46:17Z

On Tue, Jun 15, 2010 at 03:17:16PM -0400, Christoph Hellwig wrote:
> On Tue, Jun 15, 2010 at 03:13:09PM -0400, Rik van Riel wrote:
> > Why? How about because you know the stack is not big enough
> > to have the XFS call path on it twice? :)
> >
> > Isn't the whole purpose of this patch series to prevent writepage
> > from being called by the VM, when invoked from a deep callstack
> > like xfs writepage?
>
> It's not invoked from xfs writepage, but from xfs_file_aio_write via
> generic_file_buffered_write. Which isn't actually an all that deep
> callstack, just en example of one that's alread bad enough to overflow
> the stack.

Keep in mind that both ext4 and btrfs have similar checks in their
writepage path. I think Dave Chinner's stack analysis we very clear
here, there's no room in the stack for any filesystem and direct reclaim
to live happily together.

Circling back to an older thread:

> 32) 3184 64 xfs_vm_writepage+0xab/0x160 [xfs]
> 33) 3120 384 shrink_page_list+0x65e/0x840
> 34) 2736 528 shrink_zone+0x63f/0xe10
> 35) 2208 112 do_try_to_free_pages+0xc2/0x3c0
> 36) 2096 128 try_to_free_pages+0x77/0x80
> 37) 1968 240 __alloc_pages_nodemask+0x3e4/0x710
> 35) 2208 112 do_try_to_free_pages+0xc2/0x3c0
> 36) 2096 128 try_to_free_pages+0x77/0x80
> 37) 1968 240 __alloc_pages_nodemask+0x3e4/0x710
> 38) 1728 48 alloc_pages_current+0x8c/0xe0
> 39) 1680 16 __get_free_pages+0xe/0x50
> 40) 1664 48 __pollwait+0xca/0x110
> 41) 1616 32 unix_poll+0x28/0xc0
> 42) 1584 16 sock_poll+0x1d/0x20
> 43) 1568 912 do_select+0x3d6/0x700
> 44) 656 416 core_sys_select+0x18c/0x2c0
> 45) 240 112 sys_select+0x4f/0x110
> 46) 128 128 system_call_fastpath+0x16/0x1b

So, before xfs can hand this work off to one of its 16 btrees, push
it through the hand tuned irix simulator or even think about spreading
the work across 512 cpus (whoops, I guess that's just btrfs), we've used
up quite a lot of the stack.

I'm not against direct reclaim, but I think we have to admit that it has
to be done directly with another stack context. Handoff to a different
thread, whatever.

When the reclaim does happen, it would be really nice if ios were done
in large-ish clusters. Small ios reclaim less memory in more time and
slow everything down.

-chris