Re: [PATCH] acquire mmap semaphore in pagemap_read.

Matt Mackall
2009-03-17T16:24:06Z

On Tue, 2009-03-17 at 13:04 +0100, Martin Schwidefsky wrote:
> On Thu, 12 Mar 2009 16:54:51 +0100
> Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>
> > On Thu, 12 Mar 2009 10:23:34 -0500
> > Matt Mackall <mpm@selenic.com> wrote:
> >
> > > Well it means we may have to reintroduce the very annoying double
> > > buffering from various earlier implementations. But let's leave this
> > > discussion until after we've figured out what to do about the walker
> > > code.
> >
> > About the walker code. I've realized that there is another way to fix
> > this. The TASK_SIZE definition is currently used for two things: 1) as
> > a maximum mappable address, 2) the size of the address space for a
> > process. And there lies a problem: while a process is using a reduced
> > page table 1) and 2) differ. If I make TASK_SIZE give you the current
> > size of the address space then it is not possible to mmap an object
> > beyond 4TB and the page table upgrade never happens. If I make
> > TASK_SIZE return the maximum mappable address the page table walker
> > breaks. The solution could be to introduce MAX_TASK_SIZE and use that
> > in the mmap code to find out what can be mapped.
>
> I got around the TASK_SIZE checks with arch code only. In total I'll
> need two fixes, one that makes TASK_SIZE to reflect the size of the
> current address space and another one to get the page table upgrades
> working again. I'll push these patches via git390 as no common code
> changes are required.

Thanks, Martin.

> Which leaves the mmap_sem issue as the only problem left.

Yeah, this one needs more thought. I'd really rather not go back to
double-buffering here as it was much more complicated, not to mention
slow.

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