linux-kernel - Re: [PATCH v2 1/5] drivers/base/memory: Remove unneeded check in remove_memory_block

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Date:   Tue, 25 Jun 2019 10:09:14 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...een.com,
        Jonathan.Cameron@...wei.com, anshuman.khandual@....com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] drivers/base/memory: Remove unneeded check in
 remove_memory_block_devices

On Tue, Jun 25, 2019 at 10:03:31AM +0200, David Hildenbrand wrote:
> On 25.06.19 10:01, David Hildenbrand wrote:
> > On 25.06.19 09:52, Oscar Salvador wrote:
> >> remove_memory_block_devices() checks for the range to be aligned
> >> to memory_block_size_bytes, which is our current memory block size,
> >> and WARNs_ON and bails out if it is not.
> >>
> >> This is the right to do, but we do already do that in try_remove_memory(),
> >> where remove_memory_block_devices() gets called from, and we even are
> >> more strict in try_remove_memory, since we directly BUG_ON in case the range
> >> is not properly aligned.
> >>
> >> Since remove_memory_block_devices() is only called from try_remove_memory(),
> >> we can safely drop the check here.
> >>
> >> To be honest, I am not sure if we should kill the system in case we cannot
> >> remove memory.
> >> I tend to think that WARN_ON and return and error is better.
> > 
> > I failed to parse this sentence.
> > 
> >>
> >> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> >> ---
> >>  drivers/base/memory.c | 4 ----
> >>  1 file changed, 4 deletions(-)
> >>
> >> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> >> index 826dd76f662e..07ba731beb42 100644
> >> --- a/drivers/base/memory.c
> >> +++ b/drivers/base/memory.c
> >> @@ -771,10 +771,6 @@ void remove_memory_block_devices(unsigned long start, unsigned long size)
> >>  	struct memory_block *mem;
> >>  	int block_id;
> >>  
> >> -	if (WARN_ON_ONCE(!IS_ALIGNED(start, memory_block_size_bytes()) ||
> >> -			 !IS_ALIGNED(size, memory_block_size_bytes())))
> >> -		return;
> >> -
> >>  	mutex_lock(&mem_sysfs_mutex);
> >>  	for (block_id = start_block_id; block_id != end_block_id; block_id++) {
> >>  		mem = find_memory_block_by_id(block_id, NULL);
> >>
> > 
> > As I said when I introduced this, I prefer to have such duplicate checks
> > in place in case we have dependent code splattered over different files.
> > (especially mm/ vs. drivers/base). Such simple checks avoid to document
> > "start and size have to be aligned to memory blocks".
> 
> Lol, I even documented it as well. So yeah, if you're going to drop this
> once, also drop the one in create_memory_block_devices().

TBH, I would not mind sticking with it.
What sticked out the most was that in the previous check, we BUG_on while
here we just print out a warning, so it seemed quite "inconsistent" to me.

And I only stumbled upon this when I was testing a kernel module that
hot-removed memory in a different granularity.

Anyway, I do not really feel strong here, I can perfectly drop this patch as I
would rather have the focus in the following-up patches, which are the important
ones IMO.

> 
> > 
> > If you still insist, then also remove the same sequence from
> > create_memory_block_devices().
> > 
> 
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb
> 

-- 
Oscar Salvador
SUSE L3