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Message-ID: <30b3c8d3-3e2e-8f07-a05e-3c91fccde024@suse.cz>
Date:   Tue, 20 Sep 2022 12:31:42 +0200
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Subject: Re: [PATCH] mm: Make failslab writable again
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To:     Alexander Atanasov <alexander.atanasov@virtuozzo.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     kernel@openvz.org, Kees Cook <keescook@chromium.org>,
        Roman Gushchin <guro@fb.com>, Jann Horn <jannh@google.com>,
        Vijayanand Jitta <vjitta@codeaurora.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20220920082033.1727374-1-alexander.atanasov@virtuozzo.com>
 <21646f5c-39ee-a51c-f30c-272de85ee350@suse.cz>
 <6f5a8f71-3d82-97bf-90e1-0f33546bb59b@virtuozzo.com>
 <60111450-d66a-a146-1a70-0c093400f3e5@suse.cz>
 <7d79d13b-0579-452f-7a07-4e3eba205d23@virtuozzo.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <7d79d13b-0579-452f-7a07-4e3eba205d23@virtuozzo.com>
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On 9/20/22 12:21, Alexander Atanasov wrote:
> On 20.09.22 12:29, Vlastimil Babka wrote:
>> On 9/20/22 11:17, Alexander Atanasov wrote:
>>> Hello,
>>>
>>> On 20.09.22 11:42, Vlastimil Babka wrote:
>>>>> +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;
>>
>> This way also has RMW issues, and also the compiler is allowed to
>> temporarily modify s->flags any way it likes; with WRITE_ONCE() it can't.
> 
> Okay, so the safest way is this?
> 
> if (buf[0] == '1')
>     WRITE_ONCE(s->flags, READ_ONCE(s->flags) | SLAB_FAILSLAB);
> else
>     WRITE_ONCE(s->flags, READ_ONCE(s->flags) & ~SLAB_FAILSLAB);

Yeah, that would work. Given we are the only writer, we shouldn't even need
a READ_ONCE.

> It got me thinking how many places would break if the compiler
> starts to temporariliy modify the flags - i hope it never does.

That's likely true as well. But the macros have been introduced for this
purpose AFAIK.