On 09/13, Dave Hansen wrote: > > On 09/13/2016 07:59 AM, Oleg Nesterov wrote: > > I agree. I don't even understand why this was considered as a bug. > > Obviously, m_stop() which drops mmap_sep should not be called, or > > all the threads should be stopped, if you want to trust the result. > > There was a mapping at a given address. That mapping did not change, it > was not split, its attributes did not change. But, it didn't show up > when reading smaps. Folks _actually_ noticed this in a test suite > looking for that address range in smaps. I understand, and I won't argue with any change which makes the things better. Just I do not think this is a real problem. And this patch can't fix other oddities and it seems it adds another one (at least) although I can easily misread this patch and/or the code. So we change m_cache_vma(), - m->version = m_next_vma(m->private, vma) ? vma->vm_start : -1UL; + m->version = m_next_vma(m->private, vma) ? vma->vm_end : -1UL; OK, and another change in m_start() - if (vma && (vma = m_next_vma(priv, vma))) + if (vma) means that it can return the same vma if it grows in between. show_map_vma() has another change + start = max(vma->vm_start, start); so it will be reported as _another_ vma, and this doesn't look exactly right. And after that *ppos will be falsely incremented... but probably this doesn't matter because the "if (pos < mm->map_count)" logic in m_start() looks broken anyway. > IOW, we had goofy kernel behavior, and it broke a reasonable test > program. The test program just used fgets() to read into a fixed-length > buffer, which is a completely normal thing to do. > > To get "sensible results", doesn't userspace have to somehow know in > advance how many bytes of data a given VMA will generate in smaps output? Yes, /proc/has its limitations ;) Even if you read, say, /proc/pid/status you can get the corrupted result after the short read. But in this case fgets() should likely work, yes. Dave, let me repeat, I won't argue with any change and in any case you can safely ignore my opinion. Oleg. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
