Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm() and use it in fs/proc

[Yang Shi]

On 2/26/18 5:47 PM, David Rientjes wrote:
> On Mon, 26 Feb 2018, Yang Shi wrote:
>
> > > Rather than killable, we have patches that introduce down_read_unfair()
> > > variants for the files you've modified (cmdline and environ) as well as
> > > others (maps, numa_maps, smaps).
> > You mean you have such functionality used by google internally?
> Yup, see https://lwn.net/Articles/387720.
>
> > > When another thread is holding down_read() and there are queued
> > > down_write()'s, down_read_unfair() allows for grabbing the rwsem without
> > > queueing for it.  Additionally, when another thread is holding
> > > down_write(), down_read_unfair() allows for queueing in front of other
> > > threads trying to grab it for write as well.
> > It sounds the __unfair variant make the caller have chance to jump the gun to
> > grab the semaphore before other waiters, right? But when a process holds the
> > semaphore, i.e. mmap_sem, for a long time, it still has to sleep in
> > uninterruptible state, right?
> Right, it's solving two separate things which I think may be able to be
> merged together.  Killable is solving an issue where the rwsem is blocking
> for a long period of time in uninterruptible sleep, and unfair is solving
> an issue where reading the procfs files gets stalled for a long period of
> time.  We haven't run into an issue (yet) where killable would have solved
> it; we just have the unfair variants to grab the rwsem asap and then, if
> killable, gracefully return.
>
> > > Ingo would know more about whether a variant like that in upstream Linux
> > > would be acceptable.
> > >
> > > Would you be interested in unfair variants instead of only addressing
> > > killable?
> > Yes, I'm although it still looks overkilling to me for reading /proc.
> We make certain inferences on the readablility of procfs files for other
> threads to determine how much its mm's mmap_sem is contended.

I see your points here for reading /proc for system monitor. However, 
I'm concerned that the _unfair APIs get the processes which read /proc 
priority elevation (not real priority change, just look like). It might 
be abused by some applications, for example:

A high priority process and a low priority process are waiting for the 
same rwsem, if the low priority process is trying to read /proc 
maliciously on purpose, it can get elevated to grap the rwsem before any 
other processes which are waiting for the same rwsem.

Yang