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

Michel Lespinasse
2010-11-19T23:31:47Z

On Fri, Nov 19, 2010 at 2:54 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 18 Nov 2010 23:23:16 -0800
> Michel Lespinasse <walken@google.com> wrote:
>
>> On Thu, Nov 18, 2010 at 09:41:22AM -0800, Hugh Dickins wrote:
>> > On Thu, 18 Nov 2010, Christoph Hellwig wrote:
>> > > 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.
>>
>> So, I decided to test this using memtoy.
>
> Wait.  You *tested* the kernel?
>
> I dunno, kids these days...

Not guilty - I mean, Christoph made me do it !

> Dirtying all that memory at mlock() time is pretty obnoxious.
>
> I'm inclined to agree that your patch implements the desirable
> behaviour: don't dirty the page, don't do block allocation.  Take a
> fault at first-dirtying and do it then.  This does degrade mlock a bit:
> the user will find that the first touch of an mlocked page can cause
> synchronous physical I/O, which isn't mlocky behaviour *at all*.  But
> we have to be able to do this anyway - whenever the kupdate function
> writes back the dirty pages it has to mark them read-only again so the
> kernel knows when they get redirtied.

Glad to see that we seem to be coming to an agreement here.

> So all that leaves me thinking that we merge your patches as-is.  Then
> work out why users can fairly trivially use mlock to hang the kernel on
> ext2 and ext3 (and others?)

I would say the hang is not even mlock related - you see without it
also. All you need is mmap a large file with holes and write fault
pages until you run out of disk space. At that point additional write
faults wait for a writeback that can never complete. Sysadmin can
however kill -9 such processes and/or free some space, though.

--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.