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

[Jan Kara <jack@suse.cz>]

On Tue 20-11-18 11:35:13, Michal Hocko wrote:
> 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 <dan.j.williams@intel.com>
> Cc: David Rientjes <rientjes@google.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

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@suse.cz>

								Honza

> ---
>  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.
>  
>  This file is only present if the CONFIG_MMU kernel configuration option is
>  enabled.
> -- 
> 2.19.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR