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 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>
