linux-kernel - Re: [PATCH] Sys V shared memory limited to 8TiB.

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Date:	Thu, 11 Apr 2013 14:07:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Robin Holt <holt@....com>
Cc:	linux-kernel@...r.kernel.org, Alex Thorlton <athorlton@....com>
Subject: Re: [PATCH] Sys V shared memory limited to 8TiB.

On Wed, 10 Apr 2013 21:42:23 -0500 Robin Holt <holt@....com> wrote:

> On Wed, Apr 10, 2013 at 04:15:07PM -0700, Andrew Morton wrote:
> > On Tue, 9 Apr 2013 21:39:24 -0500 Robin Holt <holt@....com> wrote:
> > 
> > > Trying to run an application which was trying to put data into half
> > > of memory using shmget(), we found that having a shmall value below
> > > 8EiB-8TiB would prevent us from using anything more than 8TiB.  By setting
> > > kernel.shmall greater that 8EiB-8TiB would make the job work.
> > > 
> > > In the newseg() function, ns->shm_tot which, at 8TiB is INT_MAX.
> > 
> > You have way too much memory.
> > 
> > > ipc/shm.c:
> > >  458 static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
> > >  459 {
> > > ...
> > >  465         int numpages = (size + PAGE_SIZE -1) >> PAGE_SHIFT;
> > > ...
> > >  474         if (ns->shm_tot + numpages > ns->shm_ctlall)
> > >  475                 return -ENOSPC;
> > >
> > > ...
> > >
> > > --- a/include/linux/ipc_namespace.h
> > > +++ b/include/linux/ipc_namespace.h
> > > @@ -43,8 +43,8 @@ struct ipc_namespace {
> > >  
> > >  	size_t		shm_ctlmax;
> > >  	size_t		shm_ctlall;
> > > +	unsigned long	shm_tot;
> > >  	int		shm_ctlmni;
> > > -	int		shm_tot;
> > >  	/*
> > >  	 * Defines whether IPC_RMID is forced for _all_ shm segments regardless
> > >  	 * of shmctl()
> > 
> > I reviewed everything for fallout from this and don't see any obvious
> > issues.
> > 
> > I do wonder about the appropriateness of the unsigned long type.  Most
> > (but by no means all) code in this area uses size_t, and the
> > above-quoted ns->shm_ctlall is size_t.
> 
> The only reason I went with unsigned long instead of size_t was most
> places in the kernel track stuff I recalled that was tracking stuff
> in pages used unsigned longs.  Also, I found shm_tot field in shm_info
> structure was an unsigned long so this felt like a natural fit.  I would
> happily changed to size_t.  Whatever you feel is right.

I have no really strong feelings, but let's at least put some thought
into it.  I do prefer ulong and find size_t to be a PITA.  I guess it
doesn't matter much.

> > --- a/ipc/shm.c~ipc-sysv-shared-memory-limited-to-8tib-fix
> > +++ a/ipc/shm.c
> > @@ -462,7 +462,7 @@ static int newseg(struct ipc_namespace *
> >  	size_t size = params->u.size;
> >  	int error;
> >  	struct shmid_kernel *shp;
> > -	int numpages = (size + PAGE_SIZE -1) >> PAGE_SHIFT;
> > +	size_t numpages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
> 
> I was holding off from that change only because I was asking for this
> to go to stable and this doubles the size of the patch. ;)

It's a bug, isn't it?  Is there anything else which prevents creation
of segments which are >=8TB?

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