Re: [Xen-devel] [PATCH v6 1/5] x86/mem_sharing: reorder when pages are unlocked and released

[Jan Beulich <JBeulich@xxxxxxxx>]

On 18.07.2019 15:47, Tamas K Lengyel wrote:
> On Thu, Jul 18, 2019 at 7:33 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> On 18.07.2019 15:13, Tamas K Lengyel wrote:
>>> On Thu, Jul 18, 2019 at 7:12 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>
>>>> On 18.07.2019 14:55, Tamas K Lengyel wrote:
>>>>> On Thu, Jul 18, 2019 at 4:47 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 17.07.2019 21:33, Tamas K Lengyel wrote:
>>>>>>> @@ -900,6 +895,7 @@ static int share_pages(struct domain *sd, gfn_t 
>>>>>>> sgfn, shr_handle_t sh,
>>>>>>>          p2m_type_t smfn_type, cmfn_type;
>>>>>>>          struct two_gfns tg;
>>>>>>>          struct rmap_iterator ri;
>>>>>>> +    unsigned long put_count = 0;
>>>>>>>
>>>>>>>          get_two_gfns(sd, sgfn, &smfn_type, NULL, &smfn,
>>>>>>>                       cd, cgfn, &cmfn_type, NULL, &cmfn, 0, &tg);
>>>>>>> @@ -964,15 +960,6 @@ static int share_pages(struct domain *sd, gfn_t 
>>>>>>> sgfn, shr_handle_t sh,
>>>>>>>              goto err_out;
>>>>>>>          }
>>>>>>>
>>>>>>> -    /* Acquire an extra reference, for the freeing below to be safe. */
>>>>>>> -    if ( !get_page(cpage, dom_cow) )
>>>>>>> -    {
>>>>>>> -        ret = -EOVERFLOW;
>>>>>>> -        mem_sharing_page_unlock(secondpg);
>>>>>>> -        mem_sharing_page_unlock(firstpg);
>>>>>>> -        goto err_out;
>>>>>>> -    }
>>>>>>> -
>>>>>>>          /* Merge the lists together */
>>>>>>>          rmap_seed_iterator(cpage, &ri);
>>>>>>>          while ( (gfn = rmap_iterate(cpage, &ri)) != NULL)
>>>>>>> @@ -984,13 +971,14 @@ static int share_pages(struct domain *sd, gfn_t 
>>>>>>> sgfn, shr_handle_t sh,
>>>>>>>               * Don't change the type of rmap for the client page. */
>>>>>>>              rmap_del(gfn, cpage, 0);
>>>>>>>              rmap_add(gfn, spage);
>>>>>>> -        put_page_and_type(cpage);
>>>>>>> +        put_count++;
>>>>>>>              d = get_domain_by_id(gfn->domain);
>>>>>>>              BUG_ON(!d);
>>>>>>>              BUG_ON(set_shared_p2m_entry(d, gfn->gfn, smfn));
>>>>>>>              put_domain(d);
>>>>>>>          }
>>>>>>>          ASSERT(list_empty(&cpage->sharing->gfns));
>>>>>>> +    BUG_ON(!put_count);
>>>>>>>
>>>>>>>          /* Clear the rest of the shared state */
>>>>>>>          page_sharing_dispose(cpage);
>>>>>>> @@ -1001,7 +989,9 @@ static int share_pages(struct domain *sd, gfn_t 
>>>>>>> sgfn, shr_handle_t sh,
>>>>>>>
>>>>>>>          /* Free the client page */
>>>>>>>          put_page_alloc_ref(cpage);
>>>>>>> -    put_page(cpage);
>>>>>>> +
>>>>>>> +    while ( put_count-- )
>>>>>>> +        put_page_and_type(cpage);
>>>>>>>
>>>>>>>          /* We managed to free a domain page. */
>>>>>>>          atomic_dec(&nr_shared_mfns);
>>>>>>> @@ -1165,19 +1155,13 @@ int __mem_sharing_unshare_page(struct domain *d,
>>>>>>>          {
>>>>>>>              if ( !last_gfn )
>>>>>>>                  mem_sharing_gfn_destroy(page, d, gfn_info);
>>>>>>> -        put_page_and_type(page);
>>>>>>> +
>>>>>>>              mem_sharing_page_unlock(page);
>>>>>>> +
>>>>>>>              if ( last_gfn )
>>>>>>> -        {
>>>>>>> -            if ( !get_page(page, dom_cow) )
>>>>>>> -            {
>>>>>>> -                put_gfn(d, gfn);
>>>>>>> -                domain_crash(d);
>>>>>>> -                return -EOVERFLOW;
>>>>>>> -            }
>>>>>>>                  put_page_alloc_ref(page);
>>>>>>> -            put_page(page);
>>>>>>> -        }
>>>>>>> +
>>>>>>> +        put_page_and_type(page);
>>>>>>>              put_gfn(d, gfn);
>>>>>>>
>>>>>>>              return 0;
>>>>>>
>>>>>> ... this (main, as I guess by the title) part of the change? I think
>>>>>> you want to explain what was wrong here and/or why the new arrangement
>>>>>> is better. (I'm sorry, I guess it was this way on prior versions
>>>>>> already, but apparently I didn't notice.)
>>>>>
>>>>> It's what the patch message says - calling put_page_and_type before
>>>>> mem_sharing_page_unlock can cause a deadlock. Since now we are now
>>>>> holding a reference to the page till the end there is no need for the
>>>>> extra get_page/put_page logic when we are dealing with the last_gfn.
>>>>
>>>> The title says "reorder" without any "why".
>>>
>>> Yes, I can't reasonably fit "Calling _put_page_type while also holding
>>> the page_lock for that page can cause a deadlock." into the title. So
>>> it's spelled out in the patch message.
>>
>> Of course not. And I realize _part_ of the changes is indeed what the
>> title says (although for share_pages() that's not obvious from the
>> patch alone). But you do more: You also avoid acquiring an extra
>> reference in share_pages().
> 
> I feel like we are going in circles and having the same conversations
> over and over without really making any headway. You introduced
> grabbing the broken extra reference in 0502e0adae2. It is and was
> actually unneeded to start with if the proper solution was put in
> place, which is what this patch does, reordering things.

I'm not complaining about the changes; I'd merely like the description
state why they're needed.

>> And since you made me look at the code again: If put_page() is unsafe
>> with a lock held, how come the get_page_and_type() in share_pages()
>> is safe with two such locks held? If it really is, it surely would be
>> worthwhile to state in the description. There's another such instance
>> in mem_sharing_add_to_physmap() (plus a get_page()), and also one
>> where put_page_and_type() gets called with such a lock held (afaics).
> 
> It's possible there are other instances where this may still be
> broken. Right now I only have bandwidth to test and fix the paths I
> use. If that's unacceptable I'm happy to continue development in my
> private fork and leave things as-is upstream.

Similarly, if there are limitations - fine. But please say so in the
description, to avoid giving the impression that the issues have been
taken care of altogether.

Jan
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