Re: [PATCH] mm/vmstat: Reduce zone lock hold time when reading /proc/pagetypeinfo

[Michal Hocko]

On Thu 24-10-19 01:33:01, Qian Cai wrote:
> 
> 
> > On Oct 23, 2019, at 6:30 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > Yes, removing things is hard.  One approach is to add printk_once(this
> > is going away, please email us if you use it) then wait a few years. 
> > Backport that one-liner into -stable kernels to hopefully speed up the
> > process.
> 
> Although it still look like an overkill to me given,
> 
> 1) Mel given a green light to remove it.
> 2) Nobody justifies any sensible reason to keep it apart from it was
> probably only triggering by some testing tools blindly read procfs
> entries.

It's been useful for debugging memory fragmentation problems and we do
not have anything that would provide a similar information. Considering
that making it root only is trivial and reducing the lock hold times
likewise I do not really see any strong reason to dump it at this
moment.
 
> it is still better than wasting developersâ time to beating the âdeadâ horse.
> 
> > 
> > Meanwhile, we need to fix the DoS opportunity.  How about my suggestion
> > that we limit the count to 1024, see if anyone notices?  I bet they
> > don't!
> 
> The DoS is probably there since the file had been introduced almost 10
> years ago, so I suspect it is not that easily exploitable.

Yes you need _tons_ of memory. Reading the file on my 3TB system takes
sys     0m3.673s

The situation might be worse if the system is terribly fragmented which
is not the case here.
-- 
Michal Hocko
SUSE Labs