Re: [RFC][PATCH] mm: fix page_mkclean() vs non-linear vmas

[Peter Zijlstra]

On Thu, 2007-03-08 at 12:21 +0100, Miklos Szeredi wrote:
> > Partial revert of commit: 204ec841fbea3e5138168edbc3a76d46747cc987
> > 
> > Non-linear vmas aren't properly handled by page_mkclean() and fixing that
> > would result in linear scans of all related non-linear vmas per page_mkclean()
> > invocation.
> > 
> > This is deemed too costly, hence re-instate the msync scan for non-linear vmas.
> > 
> > However this can lead to double IO:
> > 
> >  - pages get instanciated with RO mapping
> >  - page takes write fault, and gets marked with PG_dirty
> >  - page gets tagged for writeout and calls page_mkclean()
> >  - page_mkclean() fails to find the dirty pte (and clean it)
> >  - writeout happens and PG_dirty gets cleared.
> >  - user calls msync, the dirty pte is found and the page marked with PG_dirty
> >  - the page gets writen out _again_ even though its not re-dirtied.
> > 
> > To minimize this reset the protection when creating a nonlinear vma.
> > 
> > I'm not at all happy with this, but plain disallowing
> > remap_file_pages on bdis without BDI_CAP_NO_WRITEBACK seems to
> > offend some people, hence restrict it to root only.
> 
> Root only for !BDI_CAP_NO_WRITEBACK mappings doesn't make sense
> because:
> 
>   - just encourages insecure applications
> 
>   - there are no current users that want this and presumable no future
>     uses either

AFAIK no other OS does this against regular filesystems (hear-say)

>   - it's a maintenance burden: I'll have to layer the m/ctime update
>     patch on top of this
> 
>   - the only pro for this has been that Nick thinks it cool ;)
> 
> I think the proper way to deal with this is to
> 
>   - allow BDI_CAP_NO_WRITEBACK (tmpfs/ramfs) uses, makes database
>     people happy

And UML once the remap_file_pages_prot() stuff is merged.

>   - for !BDI_CAP_NO_WRITEBACK emulate using do_mmap_pgoff(), should be
>     trivial, no userspace ABI breakage

I can live with that.

However this still leaves the non-linear reclaim (Nick pointed it out as
a potential DoS and other people have corroborated this). I have no idea
on that to do about that.

Oracle seems to mlock these things anyway, but UML surely would not.

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