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.
