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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/04/2019 15:46, George Dunlap wrote:
> 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).

Its perfectly public

http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=shortlog;h=refs/heads/x86-next

and has been in effect during commit freezes for the past several years
worth of releases.

It is fine to post a tree based on something other than xen.git/staging
so long as the cover letter identifies where the series is based.  A lot
of development already occurs in this way on xen-devel.

~Andrew

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