Re: [Xen-devel] [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 15/03/2019 18:38, Anthony PERARD wrote:
> On Fri, Mar 15, 2019 at 06:14:09PM +0000, Anthony PERARD wrote:
>> On Fri, Mar 15, 2019 at 09:58:47AM +0100, Roger Pau Monne wrote:
>>> Or if it's not possible to honor the hinted address an error is returned
>>> instead. This makes it easier to spot the actual failure, instead of
>>> failing later on when the caller of xen_remap_bucket realizes the
>>> mapping has not been created at the requested address.
>>>
>>> Also note that at least on FreeBSD using MAP_FIXED will cause mmap to
>>> try harder to honor the passed address.
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> The patch looks fine, and MAP_FIXED seems to be the expected behavior
>> even on Linux.
>>
>> Acked-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>>
>>> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
>>> @@ -185,8 +185,14 @@ static void xen_remap_bucket(MapCacheEntry *entry,
>>>      }
>>>  
>>>      if (!dummy) {
>>> +        /*
>>> +         * If the caller has requested the mapping at a specific address 
>>> use
>>> +         * MAP_FIXED to make sure it's honored. Note that with MAP_FIXED at
>>> +         * least FreeBSD will try harder to honor the passed address.
>>> +         */
>>>          vaddr_base = xenforeignmemory_map2(xen_fmem, xen_domid, vaddr,
>>> -                                           PROT_READ | PROT_WRITE, 0,
>>> +                                           PROT_READ | PROT_WRITE,
>>> +                                           vaddr ? MAP_FIXED : 0,
>>>                                             nb_pfn, pfns, err);
> 
> I've noticed that there's a mmap call just after which has also vaddr as
> hint, should that second call also have MAP_FIXED as flags?

I think so, yes. The intended usage of xen_remap_bucket() is to create a
mapcache entry - either dummy or real. So this patch fixes the present
problem for real entry now but not for dummy. In future, there might be
xen_remap_bucket() calls to create dummy entries at a predefined
location so, I think, MAP_FIXED should be passed to mmap() call as well.

Igor

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