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Violators will be prosecuted for from ; Wed, 21 Nov 2018 17:54:36 -0000 Date: Wed, 21 Nov 2018 18:54:28 +0100 From: Mike Rapoport Subject: Re: [RFC PATCH 1/3] mm, proc: be more verbose about unstable VMA flags in /proc//smaps References: <20181120103515.25280-1-mhocko@kernel.org> <20181120103515.25280-2-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181120103515.25280-2-mhocko@kernel.org> Message-Id: <20181121175427.GB5704@rapoport-lnx> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-api@vger.kernel.org, Andrew Morton , Alexey Dobriyan , linux-mm@kvack.org, LKML , Michal Hocko , Jan Kara , Dan Williams , David Rientjes On Tue, Nov 20, 2018 at 11:35:13AM +0100, Michal Hocko wrote: > From: Michal Hocko > > Even though vma flags exported via /proc//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//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 > Cc: Dan Williams > Cc: David Rientjes > Signed-off-by: Michal Hocko > --- > 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 consumer? has > +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 > -- Sincerely yours, Mike.