Re: [PATCH] Deadlock during heavy write activity to userspace NFS server on local NFS mount

[Nick Piggin <nickpiggin@yahoo.com.au>]

Avi Kivity wrote:
> Nick Piggin wrote:
> 
>> Avi Kivity wrote:
>>
>>> Nick Piggin wrote:
>>
>>
>>
>>>>> What's stopping the NFS server from ooming the machine then? Every 
>>>>> time some bit of memory becomes free, the server will consume it 
>>>>> instantly. Eventually ext3 will not be able to write anything out 
>>>>> because it is out of memory.
>>>>>
>>>> The NFS server should do the writeout a page at a time.
>>>
>>>
>>>
>>>
>>> The NFS server writes not only in response to page reclaim (as a 
>>> local NFS client), but also in response to pressure from non-local 
>>> clients. If both ext3 and NFS have the same allocation limits, NFS 
>>> may starve out ext3.
>>>
>>
>> What do you mean starve out ext3? ext3 gets written to *by the NFS 
>> server*
>> which is PF_MEMALLOC. 
> 
> 
> When the NFS server writes, it allocates pagecache and temporary 
> objects. When ext3 writes, it allocates temporary objects. If the NFS 
> server writes too much, ext3 can't allocate memory, and will never be 
> able to allocate memory.
> 

That is because your NFS server shouldn't hog as much memory as
it likes when it is PF_MEMALLOC. The entire writeout path should
do a page at a time if it is PF_MEMALLOC. Ie, the server should
be doing write, fsync.

But now that I think about it, I guess you may not be able to
distinguish that from regular writeout, so doing a page at a time
would hurt performance too much.

Hmm so I guess the idea of a per task reserve limit may be the way
to do it, yes. Thanks for bearing with me!