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 <[email protected]> wrote: >> >> > From: Thomas Gleixner <[email protected]> >> > >> > 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 <[email protected]> >> 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 <[email protected]> >> Signed-off-by: Mike Snitzer <[email protected]> >> >> 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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