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
