Re: [PATCH v4 4/5] cramfs: add mmap support

[Christoph Hellwig]

On Mon, Oct 02, 2017 at 07:33:29PM -0400, Nicolas Pitre wrote:
> On Tue, 3 Oct 2017, Richard Weinberger wrote:
> 
> > On Mon, Oct 2, 2017 at 12:29 AM, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> > > On Sun, 1 Oct 2017, Christoph Hellwig wrote:
> > >
> > >> up_read(&mm->mmap_sem) in the fault path is a still a complete
> > >> no-go,
> > >>
> > >> NAK
> > >
> > > Care to elaborate?
> > >
> > > What about mm/filemap.c:__lock_page_or_retry() then?
> > 
> > As soon you up_read() in the page fault path other tasks will race
> > with you before
> > you're able to grab the write lock.
> 
> But I _know_ that.
> 
> Could you highlight an area in my code where this is not accounted for?

Existing users of lock_page_or_retry return VM_FAULT_RETRY right after
up()ing mmap_sem, and they must already have a reference to the page
which is the only thing touched until then.

Your patch instead goes for an exclusive mmap_sem if it can, and
even if there is nothing that breaks with that scheme right now
there s nothing documenting that this actually safe, and we are
way down in the complex page fault path.