Re: [Xen-devel] [PATCH v3 1/4] x86/mem_sharing: reorder when pages are unlocked and released

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 4/29/19 3:32 PM, George Dunlap wrote:
> On 4/26/19 6:21 PM, Tamas K Lengyel wrote:
>> Calling _put_page_type while also holding the page_lock
>> for that page can cause a deadlock.
>>
>> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
>> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>> ---
>> v3: simplified patch by keeping the additional references already in-place
>> ---
>>  xen/arch/x86/mm/mem_sharing.c | 19 ++++++++++---------
>>  1 file changed, 10 insertions(+), 9 deletions(-)
>>
>> diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
>> index dfc279d371..e2f74ac770 100644
>> --- a/xen/arch/x86/mm/mem_sharing.c
>> +++ b/xen/arch/x86/mm/mem_sharing.c
>> @@ -648,10 +648,6 @@ static int page_make_private(struct domain *d, struct 
>> page_info *page)
>>          return -EBUSY;
>>      }
>>  
>> -    /* We can only change the type if count is one */
>> -    /* Because we are locking pages individually, we need to drop
>> -     * the lock here, while the page is typed. We cannot risk the 
>> -     * race of page_unlock and then put_page_type. */
>>      expected_type = (PGT_shared_page | PGT_validated | PGT_locked | 2);
>>      if ( page->u.inuse.type_info != expected_type )
>>      {
>> @@ -660,12 +656,12 @@ static int page_make_private(struct domain *d, struct 
>> page_info *page)
>>          return -EEXIST;
>>      }
>>  
>> -    /* Drop the final typecount */
>> -    put_page_and_type(page);
>> -
>>      /* Now that we've dropped the type, we can unlock */
>>      mem_sharing_page_unlock(page);
>>  
>> +    /* Drop the final typecount */
>> +    put_page_and_type(page);
>> +
>>      /* Change the owner */
>>      ASSERT(page_get_owner(page) == dom_cow);
>>      page_set_owner(page, d);
>> @@ -900,6 +896,7 @@ 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);
>> @@ -984,7 +981,7 @@ 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));
>> @@ -999,6 +996,10 @@ static int share_pages(struct domain *sd, gfn_t sgfn, 
>> shr_handle_t sh,
>>      mem_sharing_page_unlock(secondpg);
>>      mem_sharing_page_unlock(firstpg);
>>  
>> +    BUG_ON(!put_count);
>> +    while ( put_count-- )
>> +        put_page_and_type(cpage);
>> +
>>      /* Free the client page */
>>      if(test_and_clear_bit(_PGC_allocated, &cpage->count_info))
>>          put_page(cpage);
>> @@ -1167,8 +1168,8 @@ 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);
>> +        put_page_and_type(page);
>>          if ( last_gfn )
>>          {
>>              if ( !get_page(page, dom_cow) )
> 
> ...Probably should have mentioned that this needs to be applied after
> your other patch. :-)

Hmm -- actually, the base appears to be a non-publicly-available tree
(Andy's private x86-next).

I think series should:
1. Always be posted against a publicly-available tree, and
2. If that tree is not xenbits/xen.git staging, the URL and branch
should be provided.

 -George


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