Re: [PATCH 4/8] hugetlb: handle truncate racing with page faults

[Miaohe Lin]

On 2022/9/7 10:37, Mike Kravetz wrote:
> On 09/07/22 10:11, Miaohe Lin wrote:
>> On 2022/9/7 7:08, Mike Kravetz wrote:
>>> On 09/06/22 11:05, Mike Kravetz wrote:
>>>> On 09/06/22 09:48, Mike Kravetz wrote:
>>>>> On 09/06/22 15:57, Sven Schnelle wrote:
>>>>>> Hi Mike,
>>>>>>
>>>>>> Mike Kravetz <mike.kravetz@xxxxxxxxxx> writes:
>>>>>>
>>>>>>> When page fault code needs to allocate and instantiate a new hugetlb
>>>>>>> page (huegtlb_no_page), it checks early to determine if the fault is
>>>>>>> beyond i_size.  When discovered early, it is easy to abort the fault and
>>>>>>> return an error.  However, it becomes much more difficult to handle when
>>>>>>> discovered later after allocating the page and consuming reservations
>>>>>>> and adding to the page cache.  Backing out changes in such instances
>>>>>>> becomes difficult and error prone.
>>>>>>>
>>>>>>> Instead of trying to catch and backout all such races, use the hugetlb
>>>>>>> fault mutex to handle truncate racing with page faults.  The most
>>>>>>> significant change is modification of the routine remove_inode_hugepages
>>>>>>> such that it will take the fault mutex for EVERY index in the truncated
>>>>>>> range (or hole in the case of hole punch).  Since remove_inode_hugepages
>>>>>>> is called in the truncate path after updating i_size, we can experience
>>>>>>> races as follows.
>>>>>>> - truncate code updates i_size and takes fault mutex before a racing
>>>>>>>   fault.  After fault code takes mutex, it will notice fault beyond
>>>>>>>   i_size and abort early.
>>>>>>> - fault code obtains mutex, and truncate updates i_size after early
>>>>>>>   checks in fault code.  fault code will add page beyond i_size.
>>>>>>>   When truncate code takes mutex for page/index, it will remove the
>>>>>>>   page.
>>>>>>> - truncate updates i_size, but fault code obtains mutex first.  If
>>>>>>>   fault code sees updated i_size it will abort early.  If fault code
>>>>>>>   does not see updated i_size, it will add page beyond i_size and
>>>>>>>   truncate code will remove page when it obtains fault mutex.
>>>>>>>
>>>>>>> Note, for performance reasons remove_inode_hugepages will still use
>>>>>>> filemap_get_folios for bulk folio lookups.  For indicies not returned in
>>>>>>> the bulk lookup, it will need to lookup individual folios to check for
>>>>>>> races with page fault.
>>>>>>>
>>>>>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>>>>> ---
>>>>>>>  fs/hugetlbfs/inode.c | 184 +++++++++++++++++++++++++++++++------------
>>>>>>>  mm/hugetlb.c         |  41 +++++-----
>>>>>>>  2 files changed, 152 insertions(+), 73 deletions(-)
>>>>>>
>>>>>> With linux next starting from next-20220831 i see hangs with this
>>>>>> patch applied while running the glibc test suite. The patch doesn't
>>>>>> revert cleanly on top, so i checked out one commit before that one and
>>>>>> with that revision everything works.
>>>>>>
>>>>>> It looks like the malloc test suite in glibc triggers this. I cannot
>>>>>> identify a single test causing it, but instead the combination of
>>>>>> multiple tests. Running the test suite on a single CPU works. Given the
>>>>>> subject of the patch that's likely not a surprise.
>>>>>>
>>>>>> This is on s390, and the warning i get from RCU is:
>>>>>>
>>>>>> [ 1951.906997] rcu: INFO: rcu_sched self-detected stall on CPU
>>>>>> [ 1951.907009] rcu:     60-....: (6000 ticks this GP) idle=968c/1/0x4000000000000000 softirq=43971/43972 fqs=2765
>>>>>> [ 1951.907018]  (t=6000 jiffies g=116125 q=1008072 ncpus=64)
>>>>>> [ 1951.907024] CPU: 60 PID: 1236661 Comm: ld64.so.1 Not tainted 6.0.0-rc3-next-20220901 #340
>>>>>> [ 1951.907027] Hardware name: IBM 3906 M04 704 (z/VM 7.1.0)
>>>>>> [ 1951.907029] Krnl PSW : 0704e00180000000 00000000003d9042 (hugetlb_fault_mutex_hash+0x2a/0xd8)
>>>>>> [ 1951.907044]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
>>>>>> [ 1951.907095] Call Trace:
>>>>>> [ 1951.907098]  [<00000000003d9042>] hugetlb_fault_mutex_hash+0x2a/0xd8
>>>>>> [ 1951.907101] ([<00000000005845a6>] fault_lock_inode_indicies+0x8e/0x128)
>>>>>> [ 1951.907107]  [<0000000000584876>] remove_inode_hugepages+0x236/0x280
>>>>>> [ 1951.907109]  [<0000000000584a7c>] hugetlbfs_evict_inode+0x3c/0x60
>>>>>> [ 1951.907111]  [<000000000044fe96>] evict+0xe6/0x1c0
>>>>>> [ 1951.907116]  [<000000000044a608>] __dentry_kill+0x108/0x1e0
>>>>>> [ 1951.907119]  [<000000000044ac64>] dentry_kill+0x6c/0x290
>>>>>> [ 1951.907121]  [<000000000044afec>] dput+0x164/0x1c0
>>>>>> [ 1951.907123]  [<000000000042a4d6>] __fput+0xee/0x290
>>>>>> [ 1951.907127]  [<00000000001794a8>] task_work_run+0x88/0xe0
>>>>>> [ 1951.907133]  [<00000000001f77a0>] exit_to_user_mode_prepare+0x1a0/0x1a8
>>>>>> [ 1951.907137]  [<0000000000d0e42e>] __do_syscall+0x11e/0x200
>>>>>> [ 1951.907142]  [<0000000000d1d392>] system_call+0x82/0xb0
>>>>>> [ 1951.907145] Last Breaking-Event-Address:
>>>>>> [ 1951.907146]  [<0000038001d839c0>] 0x38001d839c0
>>>>>>
>>>>>> One of the hanging test cases is usually malloc/tst-malloc-too-large-malloc-hugetlb2.
>>>>>>
>>>>>> Any thoughts?
>>>>>
>>>>> Thanks for the report, I will take a look.
>>>>>
>>>>> My first thought is that this fix may not be applied,
>>>>> https://lore.kernel.org/linux-mm/Ywepr7C2X20ZvLdn@monkey/
>>>>> However, I see that that is in next-20220831.
>>>>>
>>>>> Hopefully, this will recreate on x86.
>>>>
>>>> One additional thought ...
>>>>
>>>> With this patch, we will take the hugetlb fault mutex for EVERY index in the
>>>> range being truncated or hole punched.  In the case of a very large file, that
>>>> is no different than code today where we take the mutex when removing pages
>>>> from the file.  What is different is taking the mutex for indices that are
>>>> part of holes in the file.  Consider a very large file with only one page at
>>>> the very large offset.  We would then take the mutex for each index in that
>>>> very large hole.  Depending on the size of the hole, this could appear as a
>>>> hang.
>>>>
>>>> For the above locking scheme to work, we need to take the mutex for indices
>>>> in holes in case there would happen to be a racing page fault.  However, there
>>>> are only a limited number of fault mutexes (it is a table).  So, we only really
>>>> need to take at a maximum num_fault_mutexes mutexes.  We could keep track of
>>>> these with a bitmap.
>>>>
>>>> I am not sure this is the issue you are seeing, but a test named
>>>> tst-malloc-too-large-malloc-hugetlb2 may be doing this.
>>>>
>>>> In any case, I think this issue needs to be addressed before this series can
>>>> move forward.
>>>
>>> Well, even if we address the issue of taking the same mutex multiple times,
>>
>> Can we change to take all the hugetlb fault mutex at the same time to ensure every possible
>> future hugetlb page fault will see a truncated i_size? Then we could just drop all the hugetlb
>> fault mutex before doing any heavy stuff? It seems hugetlb fault mutex could be dropped when
>> new i_size is guaranteed to be visible for any future hugetlb page fault users?
>> But I might miss something...
> 
> Yes, that is the general direction and would work well for truncation.  However,
> the same routine remove_inode_hugepages is used for hole punch, and I am pretty
> sure we want to take the fault mutex there as it can race with page faults.

Oh, sorry. I missed that case.

> 
>>
>>> this new synchronization scheme requires a folio lookup for EVERY index in
>>> the truncated or hole punched range.  This can easily 'stall' a CPU if there
>>
>> If above thought holds, we could do batch folio lookup instead. Hopes my thought will help. ;)
>>
> 
> Yes, I have some promising POC code with two batch lookups in case of holes.
> Hope to send something soon.

That will be really nice. ;)

Thanks,
Miaohe Lin