Re: [RFC PATCH 1/3] mm, proc: be more verbose about unstable VMA flags in /proc/<pid>/smaps

[Michal Hocko]

On Tue 20-11-18 16:01:47, David Rientjes wrote:
> On Tue, 20 Nov 2018, Jan Kara wrote:
> 
> > > Even though vma flags exported via /proc/<pid>/smaps are explicitly
> > > documented to be not guaranteed for future compatibility the warning
> > > doesn't go far enough because it doesn't mention semantic changes to
> > > those flags. And they are important as well because these flags are
> > > a deep implementation internal to the MM code and the semantic might
> > > change at any time.
> > > 
> > > Let's consider two recent examples:
> > > http://lkml.kernel.org/r/20181002100531.GC4135@xxxxxxxxxxxxxx
> > > : commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and device dax" has
> > > : removed VM_MIXEDMAP flag from DAX VMAs. Now our testing shows that in the
> > > : mean time certain customer of ours started poking into /proc/<pid>/smaps
> > > : and looks at VMA flags there and if VM_MIXEDMAP is missing among the VMA
> > > : flags, the application just fails to start complaining that DAX support is
> > > : missing in the kernel.
> > > 
> > > http://lkml.kernel.org/r/alpine.DEB.2.21.1809241054050.224429@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > : Commit 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
> > > : introduced a regression in that userspace cannot always determine the set
> > > : of vmas where thp is ineligible.
> > > : Userspace relies on the "nh" flag being emitted as part of /proc/pid/smaps
> > > : to determine if a vma is eligible to be backed by hugepages.
> > > : Previous to this commit, prctl(PR_SET_THP_DISABLE, 1) would cause thp to
> > > : be disabled and emit "nh" as a flag for the corresponding vmas as part of
> > > : /proc/pid/smaps.  After the commit, thp is disabled by means of an mm
> > > : flag and "nh" is not emitted.
> > > : This causes smaps parsing libraries to assume a vma is eligible for thp
> > > : and ends up puzzling the user on why its memory is not backed by thp.
> > > 
> > > In both cases userspace was relying on a semantic of a specific VMA
> > > flag. The primary reason why that happened is a lack of a proper
> > > internface. While this has been worked on and it will be fixed properly,
> > > it seems that our wording could see some refinement and be more vocal
> > > about semantic aspect of these flags as well.
> > > 
> > > Cc: Jan Kara <jack@xxxxxxx>
> > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > Honestly, it just shows that no amount of documentation is going to stop
> > userspace from abusing API that's exposing too much if there's no better
> > alternative. But this is a good clarification regardless. So feel free to
> > add:
> > 
> > Acked-by: Jan Kara <jack@xxxxxxx>
> > 
> 
> I'm not sure what is expected of a userspace developer who finds they have 
> a single way to determine if something is enabled/disabled.  Should they 
> refer to the documentation and see that the flag may be unstable so they 
> write a kernel patch and have it merged upstream before using it?  What to 
> do when they don't control the kernel version they are running on?

Well, I would treat it as any standard feature request. Ask for the
feature upstream and work with the comunity to come up with a reasonable
and a stable API.

> Anyway, mentioning that the vm flags here only have meaning depending on 
> the kernel version seems like a worthwhile addition:
> 
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

Thanks!

-- 
Michal Hocko
SUSE Labs