Re: [Bug 9182] Critical memory leak (dirty pages)

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Friday 21 December 2007 06:24, Linus Torvalds wrote:
> On Thu, 20 Dec 2007, Jan Kara wrote:
> >   As I wrote in my previous email, this solution works but hides the
> > fact that the page really *has* dirty data in it and *is* pinned in
> > memory until the commit code gets to writing it. So in theory it could
> > disturb the writeout logic by having more dirty data in memory than vm
> > thinks it has. Not that I'd have a better fix now but I wanted to point
> > out this problem.
>
> Well, I worry more about the VM being sane - and by the time we actually
> hit this case, as far as VM sanity is concerned, the page no longer really
> exists. It's been removed from the page cache, and it only really exists
> as any other random kernel allocation.

It does allow the VM to just not worry about this. However I don't
really like this kinds of catch-all conditions that are hard to get
rid of and can encourage bad behaviour.

It would be nice if the "insane" things were made to clean up after
themselves.


> The fact that low-level filesystems (in this case ext3 journaling) do
> their own insane things is not something the VM even _should_ care about.
> It's just an internal FS allocation, and the FS can do whatever the hell
> it wants with it, including doing IO etc.
>
> The kernel doesn't consider any other random IO pages to be "dirty" either
> (eg if you do direct-IO writes using low-level SCSI commands, the VM
> doesn't consider that to be any special dirty stuff, it's just random page
> allocations again). This is really no different.
>
> In other words: the Linux "VM" subsystem is really two differnt parts: the
> low-level page allocator (which obviously knows that the page is still in
> *use*, since it hasn't been free'd), and the higher-level file mapping and
> caching stuff that knows about things like page "dirtyiness". And once
> you've done a "remove_from_page_cache()", the higher levels are no longer
> involved, and dirty accounting simply doesn't get into the picture.

That's all true... it would simply be nice to ask the filesystems to do
this. But anyway I think your patch is pretty reasonable for the moment.