Re: [PATCH] acquire mmap semaphore in pagemap_read.

Martin Schwidefsky
2009-03-17T12:10:27Z

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.

Which leaves the mmap_sem issue as the only problem left.