Re: [PATCH] ipc,shm: disable shmmax and shmall by default

[Davidlohr Bueso]

On Fri, 2014-04-11 at 20:28 +0200, Manfred Spraul wrote:
> Hi Davidlohr,
> 
> On 04/03/2014 02:20 AM, Davidlohr Bueso wrote:
> > The default size for shmmax is, and always has been, 32Mb.
> > Today, in the XXI century, it seems that this value is rather small,
> > making users have to increase it via sysctl, which can cause
> > unnecessary work and userspace application workarounds[1].
> >
> > [snip]
> > Running this patch through LTP, everything passes, except the following,
> > which, due to the nature of this change, is quite expected:
> >
> > shmget02    1  TFAIL  :  call succeeded unexpectedly
> Why is this TFAIL expected?

So looking at shmget02.c, this is the case that fails:

		for (i = 0; i < TST_TOTAL; i++) {
			/*
			 * Look for a failure ...
			 */

			TEST(shmget(*(TC[i].skey), TC[i].size, TC[i].flags));

			if (TEST_RETURN != -1) {
				tst_resm(TFAIL, "call succeeded unexpectedly");
				continue;
			}

Where TC[0] is: 
struct test_case_t {
	int *skey;
	int size;
	int flags;
	int error;
} TC[] = {
	/* EINVAL - size is 0 */
	{
	&shmkey2, 0, IPC_CREAT | IPC_EXCL | SHM_RW, EINVAL},

So it's expected because now 0 is actually valid. And before:

 EINVAL A new segment was to be created and size < SHMMIN or size > SHMMAX

> >
> > diff --git a/ipc/shm.c b/ipc/shm.c
> > index 7645961..ae01ffa 100644
> > --- a/ipc/shm.c
> > +++ b/ipc/shm.c
> > @@ -490,10 +490,12 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
> >   	int id;
> >   	vm_flags_t acctflag = 0;
> >   
> > -	if (size < SHMMIN || size > ns->shm_ctlmax)
> > +	if (ns->shm_ctlmax &&
> > +	    (size < SHMMIN || size > ns->shm_ctlmax))
> >   		return -EINVAL;
> >   
> > -	if (ns->shm_tot + numpages > ns->shm_ctlall)
> > +	if (ns->shm_ctlall &&
> > +	    ns->shm_tot + numpages > ns->shm_ctlall)
> >   		return -ENOSPC;
> >   
> >   	shp = ipc_rcu_alloc(sizeof(*shp));
> Ok, I understand it:
> Your patch disables checking shmmax, shmall *AND* checking for SHMMIN.

Right, if shmmax is 0, then there's no point checking for shmmin,
otherwise we'd always end up returning EINVAL.

> 
> a) Have you double checked that 0-sized shm segments work properly?
>   Does the swap code handle it properly, ...? EINVAL A new segment was to be created and size < SHMMIN or size > SHMMAX

Hmm so I've been using this patch just fine on my laptop since I sent
it. So far I haven't seen any issues. Are you refering to something in
particular? I'd be happy to run any cases you're concerned with.

> b) It's that yet another risk for user space incompatibility?

Sorry, I don't follow here.

> c) The patch summary is misleading, the impact on SHMMIN is not mentioned.

Sure, I can explicitly add it to the changelog.

Thanks,
Davidlohr


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