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

[Mike Rapoport]

On Wed, Nov 21, 2018 at 08:05:00AM +0100, Michal Hocko wrote:
> On Tue 20-11-18 10:32:07, Dan Williams wrote:
> > On Tue, Nov 20, 2018 at 2:35 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > >
> > > 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>
> > > ---
> > >  Documentation/filesystems/proc.txt | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> > > index 12a5e6e693b6..b1fda309f067 100644
> > > --- a/Documentation/filesystems/proc.txt
> > > +++ b/Documentation/filesystems/proc.txt
> > > @@ -496,7 +496,9 @@ flags associated with the particular virtual memory area in two letter encoded
> > >
> > >  Note that there is no guarantee that every flag and associated mnemonic will
> > >  be present in all further kernel releases. Things get changed, the flags may
> > > -be vanished or the reverse -- new added.
> > > +be vanished or the reverse -- new added. Interpretatation of their meaning
> > > +might change in future as well. So each consumnent of these flags have to
> > > +follow each specific kernel version for the exact semantic.
> > 
> > Can we start to claw some of this back? Perhaps with a config option
> > to hide the flags to put applications on notice?
> 
> I would love to. My knowledge of CRIU is very minimal, but my
> understanding is that this is the primary consumer of those flags. And
> checkpointing is so close to the specific kernel version that I assume
> that this abuse is somehow justified.

CRIU relies on vmflags to recreate exactly the same address space layout at
restore time.

> We can hide it behind CONFIG_CHECKPOINT_RESTORE but does it going to
> help? I presume that many distro kernels will have the config enabled.

They do :)
 
> > I recall that when I
> > introduced CONFIG_IO_STRICT_DEVMEM it caused enough regressions that
> > distros did not enable it, but now a few years out I'm finding that it
> > is enabled in more places.
> > 
> > In any event,
> > 
> > Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Forgot that in my previous nit-picking e-mail:

Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
 
> Thanks!
> 
> -- 
> Michal Hocko
> SUSE Labs
> 

-- 
Sincerely yours,
Mike.