Re: 2.6.6-rc{1,2} bad VM/NFS interaction in case of dirty pagewriteback

[Andrew Morton]

Shantanu Goel <sgoel01@xxxxxxxxx> wrote:
>
> Hi,
> 
> During page reclamation when the scanner encounters a
> dirty page and invokes writepage(), if the FS layer
> returns WRITEPAGE_ACTIVATE as NFS does, I think the
> page should not be placed on the active as is
> presently done.  This can cause a lot of extraneous
> swapout activity because in the presence of a large
> active list, the pages being written out will not be
> reclaimed quickly enough.  It also seems counter
> intuitive since the scanner has just determined that
> the page has not been recently referenced.
> 
> Shouldn't the following code from shrink_list():
> 
> res = mapping->a_ops->writepage(page, &wbc);
> if (res < 0)
> 	handle_write_error(mapping, page, res);
> if (res == WRITEPAGE_ACTIVATE) {
> 	ClearPageReclaim(page);
> 	goto activate_locked;
> }
> 
> read:
> 
> res = mapping->a_ops->writepage(page, &wbc);
> if (res < 0)
> 	handle_write_error(mapping, page, res);
> if (res == WRITEPAGE_ACTIVATE) {
> 	ClearPageReclaim(page);
> 	goto keep_locked;
> }

WRITEPAGE_ACTIVATE is a bit of a hack to fix up specific peculiarities of
the interaction between tmpfs and page reclaim.

Trond, the changelog for that patch does not explain what is going on in
there - can you help out?

Also, what's the theory behind the handling of BDI_write_congested and
nfs_wait_on_write_congestion() in nfs_writepages()?  From a quick peek it
looks like NFS should be waking the sleepers in blk_congestion_wait()
rather than doing it privately?

> I can observe the benefit of this change if I run a dd
> on an NFS mount with the active list full of mostly
> mapped pages.  The stock kernel ends up paging out
> quite a bit of memory whereas the modified kernel does
> not.

yup.  We should be able to handle the throttling and writeback scheduling
from within core VFS/VM.  NFS should set and clear the backing_dev
congestion state appropriately and the VFS should take care of the rest. 
The missing bit is the early blk_congestion_wait() termination.

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