Re: [patch V2 7/8] dm verity fec: Check result of init_rs()

[NeilBrown <neilb@suse.com>]

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On Thu, Apr 19 2018, Thomas Gleixner wrote:

> On Thu, 19 Apr 2018, Mike Snitzer wrote:
>
>> On Thu, Apr 19 2018 at  6:04am -0400,
>> Thomas Gleixner <tglx@linutronix.de> wrote:
>> 
>> > From: Thomas Gleixner <tglx@linutronix.de>
>> > 
>> > The allocation of the reed solomon control structure can fail, but
>> > fec_alloc_bufs() ignores that and subsequent operations in dm verity use
>> > the potential NULL pointer unconditionally.
>> > 
>> > Add a proper check and abort if init_rs() fails.
>> 
>> This changelog makes little sense: init_rs() isn't in play relative to
>> this patch.
>
> 	fio->rs = mempool_alloc(v->fec->rs_pool, GFP_NOIO);
>
>         f->rs_pool = mempool_create(num_online_cpus(), fec_rs_alloc,
>                                     fec_rs_free, (void *) v);
>
> static void *fec_rs_alloc(gfp_t gfp_mask, void *pool_data)
> {
> 	struct dm_verity *v = (struct dm_verity *)pool_data;
>
>         return init_rs(8, 0x11d, 0, 1, v->fec->roots);
> }
>
> So init_rs() is part of the chain, right?
>
> Yes. I missed the NOIO part. But....
>
>> And it runs counter to this commit's changelog:
>> 
>> commit 34c96507e8f6be497c15497be05f489fb34c5880
>> Author: NeilBrown <neilb@suse.com>
>> Date:   Mon Apr 10 12:13:00 2017 +1000
>> 
>>     dm verity fec: fix GFP flags used with mempool_alloc()
>> 
>>     mempool_alloc() cannot fail for GFP_NOIO allocation, so there is no
>>     point testing for failure.
>> 
>>     One place the code tested for failure was passing "0" as the GFP
>>     flags.  This is most unusual and is probably meant to be GFP_NOIO,
>>     so that is changed.
>> 
>>     Also, allocation from ->extra_pool and ->prealloc_pool are repeated
>>     before releasing the previous allocation.  This can deadlock if the code
>>     is servicing a write under high memory pressure.  To avoid deadlocks,
>>     change these to use GFP_NOWAIT and leave the error handling in place.
>> 
>>     Signed-off-by: NeilBrown <neilb@suse.com>
>>     Signed-off-by: Mike Snitzer <snitzer@redhat.com>
>> 
>> Seems there is no real need for this patch.  Neil, what do you think?

I think the code is correct as-is.

>
> The analysis above forgot to look at the mempool->alloc() callback. So yes,
> while the NOIO is good at the mempool level, but init_rs() uses GPF_KERNEL
> so there might be a different can of wurms lurking.

The ->alloc call back is not relevant to the question of when
mempool_alloc() can return NULL.
If the ->alloc() callback returns a non-NULL value, it will be returned
by mempool_alloc().
If it returns NULL, that will not be returned.

mempool_alloc() *only* returns NULL in one place:

	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
		spin_unlock_irqrestore(&pool->lock, flags);
		return NULL;
	}

so a NULL return is purely dependent on the GFP flags passed.
GFP_NOIO contains __GFP_DIRECT_RECLAIM, so NULL cannot be returned.

It seems quite broken that init_rs() uses GFP_KERNEL. It should take a
gfp_t arg for the allocation.
If the mempool_alloc() above really needs GFP_NOIO, then it could
theoretically deadlock as it performs a GFP_KERNEL allocation inside
rs_init().  So in that sense, the code is not correct as-is.
It could possibly be fixed by calling memalloc_noio_save() /
memalloc_noio_restore() around the call to init_rs() in fec_rs_alloc().


Thanks,
NeilBrown

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