Re: [PATCH] mm: Make failslab writable again

[Alexander Atanasov <alexander.atanasov@virtuozzo.com>]

Hello,

On 20.09.22 11:42, Vlastimil Babka wrote:
> On 9/20/22 10:20, Alexander Atanasov wrote:
>> In (060807f841ac mm, slub: make remaining slub_debug related attributes
>> read-only failslab) it was made RO.
> 
> "read-only) failslab was made RO" ?

Yep.

>> I think it became a collateral victim to the other two options
>> (sanity_checks and trace) for which the reasons are perfectly valid.
> 
> The commit also mentioned that modifying the flags is not protected in any
> way, see below.

Yes, indeed.

>> +static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
>> +				size_t length)
>> +{
>> +	if (s->refcount > 1)
>> +		return -EINVAL;
>> +
>> +	s->flags &= ~SLAB_FAILSLAB;
>> +	if (buf[0] == '1')
>> +		s->flags |= SLAB_FAILSLAB;
> 
> Could we at least use a temporary variable to set up the final value and
> then do a WRITE_ONCE() to s->flags, so the compiler is not allowed to do
> some funky stuff? Assuming this is really the only place where we modify
> s->flags during runtime, so we can't miss other updates due to RMW.

Since it is set or clear - instead of temporary variable and potentially 
two writes and RMW issues i would suggest this:
+	if (buf[0] == '1')
+		s->flags |= SLAB_FAILSLAB;
+       else
+		s->flags &= ~SLAB_FAILSLAB;

If at some point more places need to modify the flags at runtime they 
can switch to atomic bit ops.

-- 
Regards,
Alexander Atanasov