Re: xc_gntshr_unmap problems (BUG(s) in xen-gntalloc?)

[David Vrabel <david.vrabel@citrix.com>]

On 29/08/14 13:40, Dave Scott wrote:
> Hi,
> 
> On 28 Aug 2014, at 14:50, David Vrabel <david.vrabel@citrix.com> wrote:
> 
>> On 27/08/14 22:33, Dave Scott wrote:
>>> I notice xc_gntshr_munmap for Linux simply calls 'munmap'
>>>
>>> static int linux_gntshr_munmap(xc_gntshr *xcg, xc_osdep_handle h,
>>>                               void *start_address, uint32_t count)
>>> {
>>>    return munmap(start_address, count);
>>> }
>>
>> munmap() needs a byte length, not a page count.
>>
>> When using xc_gntshr_munmap() with multiple pages this results in none
>> of the grefs being deleted (unshared and freed) since a mapping to some
>> of the grefs in the set remain.
> 
> Aha, good spot. I worked around this in my test program by calling xc_gntshr_munmap with (count * 4096) and it’s no longer leaking.
> 
>> This doesn't appear to explain why they're not deleted by the device is
>> closed.
> 
> After rebuilding xen-gntalloc with your change I couldn’t reproduce this. I tried all combinations of
> 
>   * map 1 or 2 pages
>   * unmap 0, 1 or 2 pages
> 
> and, although some of the iterations did run out of grant references (as expected), nothing seemed to be leaked over a close.

Ok,  Are you going to submit a libxc patch fixing the munmap() length?

>>> -- so I guess the problem is with the xen-gntalloc driver?
>>>
>>> If I share single pages at a time then it triggers a BUG:
>>> $ sudo ./test-gnt 1
>>> [  148.564281] BUG: unable to handle kernel paging request at ffffc908001bff20
>>> [  148.564299] IP: [<ffffffff813acf93>] gnttab_query_foreign_access+0x13/0x20
>>> [  148.564312] PGD 3d520067 PUD 0
>>> [  148.564317] Oops: 0000 [#1] SMP
>>> [  148.564322] CPU 0
>>> [  148.564325] Modules linked in: xenfs xen_evtchn xen_gntalloc xen_gntdev lp parport
>>> [  148.564337]
>>> [  148.564340] Pid: 897, comm: test-gnt Not tainted 3.2.0-67-generic #101-Ubuntu
>>> [  148.564348] RIP: e030:[<ffffffff813acf93>]  [<ffffffff813acf93>] gnttab_query_foreign_access+0x13/0x20
>>> [  148.564356] RSP: e02b:ffff88003c655da0  EFLAGS: 00010286
>>> [  148.564360] RAX: ffffc900001c0000 RBX: ffff88003cdb9e40 RCX: 0000000000000000
>>> [  148.564365] RDX: 0000000000000000 RSI: 000000000007026e RDI: 00000000ffffffe4
>>> [  148.564371] RBP: ffff88003c655dd8 R08: 0000000000000000 R09: 000000000003725f
>>> [  148.564376] R10: ffffea0000ef3680 R11: 0000000000000000 R12: ffff88003cdb9e40
>>> [  148.564381] R13: 0000000000000000 R14: ffff88003c655e80 R15: 0000000000000000
>>> [  148.564389] FS:  00007ffe79406740(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000
>>> [  148.564394] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
>>> [  148.564400] CR2: ffffc908001bff20 CR3: 000000003cdc6000 CR4: 0000000000000660
>>> [  148.564406] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> [  148.564412] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>>> [  148.564418] Process test-gnt (pid: 897, threadinfo ffff88003c654000, task ffff88003cdd4500)
>>> [  148.564423] Stack:
>>> [  148.564426]  ffffffffa000d1a5 ffff88003c655dd8 ffffffff813adbdb 00000000ffffffe4
>>> [  148.564435]  0000000000000000 00000000ffffffe4 ffff88003cdb9e40 ffff88003c655e68
>>> [  148.564443]  ffffffffa000d848 ffff88003cc47790 ffff88003c5a8dc0 ffff8800041aeba8
>>> [  148.564452] Call Trace:
>>> [  148.564459]  [<ffffffffa000d1a5>] ? __del_gref+0x105/0x150 [xen_gntalloc]
>>> [  148.564465]  [<ffffffff813adbdb>] ? gnttab_grant_foreign_access+0x2b/0x80
>>> [  148.564471]  [<ffffffffa000d848>] add_grefs+0x1c8/0x2b0 [xen_gntalloc]
>>> [  148.564478]  [<ffffffffa000da28>] gntalloc_ioctl_alloc+0xf8/0x160 [xen_gntalloc]
>>> [  148.564485]  [<ffffffffa000dae0>] gntalloc_ioctl+0x50/0x64 [xen_gntalloc]
>>> [  148.564492]  [<ffffffff8118d45a>] do_vfs_ioctl+0x8a/0x340
>>> [  148.564498]  [<ffffffff811456b3>] ? do_munmap+0x1f3/0x2f0
>>> [  148.564504]  [<ffffffff8118d7a1>] sys_ioctl+0x91/0xa0
>>> [  148.564510]  [<ffffffff8166bd42>] system_call_fastpath+0x16/0x1b
>>> [  148.564515] Code: f8 48 8b 15 98 89 b6 00 66 89 04 fa 5d c3 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 66 66 66 66 90 48 8b 05 78 89 b6 00 89 ff 5d <0f> b7 04 f8 83 e0 18 c3 0f 1f 44 00 00 55 48 89 e5 66 66 66 66
>>> [  148.564577] RIP  [<ffffffff813acf93>] gnttab_query_foreign_access+0x13/0x20
>>> [  148.564583]  RSP <ffff88003c655da0>
>>> [  148.564586] CR2: ffffc908001bff20
>>> [  148.564591] ---[ end trace 57b3a513f0d79bd6 ]---
>>
>> Does this patch fix the oops?
> 
> Yes, I’ve left my test case running for several hours with no sign of trouble.

Thanks. I'll take that as a

Tested-by: Dave Scott <david.scott@citrix.com>

Konrad, Boris, can you review please?

David

>> 8<-------------------------------------
>> xen/gntalloc: safely delete grefs in add_grefs() undo path
>>
>> If a gref could not be added (perhaps because the limit has been
>> reached or there are no more grant references available).  The undo
>> path may crash because __del_gref() frees the gref while it is being
>> used for a list iteration.
>>
>> A comment suggests that using list_for_each_entry() is safe since the
>> gref isn't removed from the list being iterated over, but it is freed
>> and thus list_for_each_entry_safe() must be used.
>>
>> Also, explicitly delete the gref from the per-file list, even though
>> this is not strictly necessary.
>>
>> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
>> ---
>> drivers/xen/gntalloc.c |    6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
>> index 787d179..b8af1ba 100644
>> --- a/drivers/xen/gntalloc.c
>> +++ b/drivers/xen/gntalloc.c
>> @@ -124,7 +124,7 @@ static int add_grefs(struct
>> ioctl_gntalloc_alloc_gref *op,
>> 	int i, rc, readonly;
>> 	LIST_HEAD(queue_gref);
>> 	LIST_HEAD(queue_file);
>> -	struct gntalloc_gref *gref;
>> +	struct gntalloc_gref *gref, *next;
>>
>> 	readonly = !(op->flags & GNTALLOC_FLAG_WRITABLE);
>> 	rc = -ENOMEM;
>> @@ -162,9 +162,9 @@ undo:
>> 	mutex_lock(&gref_mutex);
>> 	gref_size -= (op->count - i);
>>
>> -	list_for_each_entry(gref, &queue_file, next_file) {
>> -		/* __del_gref does not remove from queue_file */
>> +	list_for_each_entry_safe(gref, next, &queue_file, next_file) {
>> 		__del_gref(gref);
>> +		list_del(&gref->next_file);
>> 	}
>>
>> 	/* It's possible for the target domain to map the just-allocated grant