Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps

[Xiao Guangrong]

On 09/13/2016 03:10 AM, Michal Hocko wrote:
> On Mon 12-09-16 08:01:06, Dave Hansen wrote:
> > On 09/12/2016 05:54 AM, Michal Hocko wrote:
> > > > > In order to fix this bug, we make 'file->version' indicate the end address
> > > > > of current VMA
> > > Doesn't this open doors to another weird cases. Say B would be partially
> > > unmapped (tail of the VMA would get unmapped and reused for a new VMA.
> >
> > In the end, this interface isn't about VMAs.  It's about addresses, and
> > we need to make sure that the _addresses_ coming out of it are sane.  In
> > the case that a VMA was partially unmapped, it doesn't make sense to
> > show the "new" VMA because we already had some output covering the
> > address of the "new" VMA from the old one.
>
> OK, that is a fair point and it speaks for caching the vm_end rather
> than vm_start+skip.
>
> > > I am not sure we provide any guarantee when there are more read
> > > syscalls. Hmm, even with a single read() we can get inconsistent results
> > > from different threads without any user space synchronization.
> >
> > Yeah, very true.  But, I think we _can_ at least provide the following
> > guarantees (among others):
> > 1. addresses don't go backwards
> > 2. If there is something at a given vaddr during the entirety of the
> >    life of the smaps walk, we will produce some output for it.
>
> I guess we also want
>   3. no overlaps with previously printed values (assuming two subsequent
>      reads without seek).
>
> the patch tries to achieve the last part as well AFAICS but I guess this
> is incomplete because at least /proc/<pid>/smaps will report counters
> for the full vma range while the header (aka show_map_vma) will report
> shorter (non-overlapping) range. I haven't checked other files which use
> m_{start,next}

You are right. Will fix both /proc/PID/smaps and /proc/PID/maps in
the next version.

> Considering how this all can be tricky and how partial reads can be
> confusing and even misleading I am really wondering whether we
> should simply document that only full reads will provide a sensible
> results.

Make sense. Will document the guarantee in
Documentation/filesystems/proc.txt

Thank you, Dave and Michal, for figuring out the right direction. :)