Re: [HELP] FUSE writeback performance bottleneck

[Jingbo Xu <jefflexu@linux.alibaba.com>]

Hi Bernd and Miklos,

On 6/3/24 11:19 PM, Miklos Szeredi wrote:
> On Mon, 3 Jun 2024 at 16:43, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
>>
>>
>>
>> On 6/3/24 08:17, Jingbo Xu wrote:
>>> Hi, Miklos,
>>>
>>> We spotted a performance bottleneck for FUSE writeback in which the
>>> writeback kworker has consumed nearly 100% CPU, among which 40% CPU is
>>> used for copy_page().
>>>
>>> fuse_writepages_fill
>>>   alloc tmp_page
>>>   copy_highpage
>>>
>>> This is because of FUSE writeback design (see commit 3be5a52b30aa
>>> ("fuse: support writable mmap")), which newly allocates a temp page for
>>> each dirty page to be written back, copy content of dirty page to temp
>>> page, and then write back the temp page instead.  This special design is
>>> intentional to avoid potential deadlocked due to buggy or even malicious
>>> fuse user daemon.
>>
>> I also noticed that and I admin that I don't understand it yet. The commit says
>>
>> <quote>
>>     The basic problem is that there can be no guarantee about the time in which
>>     the userspace filesystem will complete a write.  It may be buggy or even
>>     malicious, and fail to complete WRITE requests.  We don't want unrelated parts
>>     of the system to grind to a halt in such cases.
>> </quote>
>>
>>
>> Timing - NFS/cifs/etc have the same issue? Even a local file system has no guarantees
>> how fast storage is?
> 
> I don't have the details but it boils down to the fact that the
> allocation context provided by GFP_NOFS (PF_MEMALLOC_NOFS) cannot be
> used by the unprivileged userspace server (and even if it could,
> there's no guarantee, that it would).
> 
> When this mechanism was introduced, the deadlock was a real
> possibility.  I'm not sure that it can still happen, but proving that
> it cannot might be difficult.

IIUC, there are two sources that may cause deadlock:
1) the fuse server needs memory allocation when processing FUSE_WRITE
requests, which in turn triggers direct memory reclaim, and FUSE
writeback then - deadlock here
2) a process that trigfgers direct memory reclaim or calls sync(2) may
hang there forever, if the fuse server is buggyly or malicious and thus
hang there when processing FUSE_WRITE requests

Thus the temp page design was introduced to avoid the above potential
issues.

I think case 1 may be fixed (if any), but I don't know how case 2 can be
avoided as any one could run a fuse server in unprivileged mode.  Or if
case 2 really matters?  Please correct me if I miss something.

-- 
Thanks,
Jingbo