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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Apr 29, 2019 at 8:54 AM George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
>
> On 4/29/19 3:49 PM, Andrew Cooper wrote:
> > 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
>
> OK, didn't realize this.
>
> >
> > 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.
>
> I agree it's fine when a URL is provided.
>

Yeap, sorry, should have pointed out I rebased on x86-next so I don't
keep sending patches that are already pulled in.

Tamas

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