Messages in this thread | | | Subject | Re: [PATCH] Fix region lost in /proc/self/smaps | From | Xiao Guangrong <> | Date | Thu, 8 Sep 2016 11:36:11 +0800 |
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On 09/08/2016 12:34 AM, Dave Hansen wrote: > On 09/06/2016 11:51 PM, Xiao Guangrong wrote: >> In order to fix this bug, we make 'file->version' indicate the next VMA >> we want to handle > > This new approach makes it more likely that we'll skip a new VMA that > gets inserted in between the read()s. But, I guess that's OK. We don't > exactly claim to be giving super up-to-date data at the time of read().
Yes, I completely agree with you. :)
> > With the old code, was there also a case that we could print out the > same virtual address range more than once? It seems like that could > happen if we had a VMA split between two reads.
Yes.
> > I think this introduces one oddity: if you have a VMA merge between two > reads(), you might get the same virtual address range twice in your > output. This didn't happen before because we would have just skipped > over the area that got merged. > > Take two example VMAs: > > vma-A: (0x1000 -> 0x2000) > vma-B: (0x2000 -> 0x3000) > > read() #1: prints vma-A, sets m->version=0x2000 > > Now, merge A/B to make C: > > vma-C: (0x1000 -> 0x3000) > > read() #2: find_vma(m->version=0x2000), returns vma-C, prints vma-C > > The user will see two VMAs in their output: > > A: 0x1000->0x2000 > C: 0x1000->0x3000 > > Will it confuse them to see the same virtual address range twice? Or is > there something preventing that happening that I'm missing? >
You are right. Nothing can prevent it.
However, it is not easy to handle the case that the new VMA overlays with the old VMA already got by userspace. I think we have some choices: 1: One way is completely skipping the new VMA region as current kernel code does but i do not think this is good as the later VMAs will be dropped.
2: show the un-overlayed portion of new VMA. In your case, we just show the region (0x2000 -> 0x3000), however, it can not work well if the VMA is a new created region with different attributions.
3: completely show the new VMA as this patch does.
Which one do you prefer?
Thanks!
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