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

On Wed, Nov 21, 2018 at 08:05:00AM +0100, Michal Hocko wrote:

On Tue 20-11-18 10:32:07, Dan Williams wrote:

quoted

On Tue, Nov 20, 2018 at 2:35 AM Michal Hocko [off-list ref] wrote:

quoted

From: Michal Hocko <mhocko@suse.com>

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@quack2.suse.cz
: 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@chino.kir.corp.google.com
: 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@suse.cz>
Cc: Dan Williams <redacted>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 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 :)
 

quoted

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 <redacted>

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

Acked-by: Mike Rapoport <redacted>
 

Thanks!

-- 
Michal Hocko
SUSE Labs

-- 
Sincerely yours,
Mike.