Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

Hugh Dickins
2010-11-18T17:41:37Z

On Thu, 18 Nov 2010, Christoph Hellwig wrote:
> On Thu, Nov 18, 2010 at 10:11:43AM +1100, Dave Chinner wrote:
> > Hence I think that avoiding ->page_mkwrite callouts is likely to
> > break some filesystems in subtle, undetected ways. IMO, regardless
> > of what is done, it would be really good to start by writing a new
> > regression test to exercise and encode the expected the mlock
> > behaviour so we can detect regressions later on....
>
> I think it would help if we could drink a bit of the test driven design
> coolaid here. Michel, can you write some testcases where pages on a
> shared mapping are mlocked, then dirtied and then munlocked, and then
> written out using msync/fsync. Anything that fails this test on
> btrfs/ext4/gfs/xfs/etc obviously doesn't work.

Whilst it's hard to argue against a request for testing, Dave's worries
just sprang from a misunderstanding of all the talk about "avoiding ->
page_mkwrite". There's nothing strange or risky about Michel's patch,
it does not avoid ->page_mkwrite when there is a write: it just stops
pretending that there was a write when locking down the shared area.

Hugh