Messages in this thread | | | Subject | Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps | From | Xiao Guangrong <> | Date | Tue, 13 Sep 2016 11:01:09 +0800 |
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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. :)
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