On Tue 11-07-17 09:58:42, Anshuman Khandual wrote: > On 07/10/2017 07:19 PM, Michal Hocko wrote: > > On Mon 10-07-17 16:40:59, Anshuman Khandual wrote: > >> As 'delta' is an unsigned long, 'end' (vma->vm_end + delta) cannot > >> be less than 'vma->vm_end'. > > > > This just doesn't make any sense. This is exactly what the overflow > > check is for. Maybe vm_end + delta can never overflow because of > > (old_len == vma->vm_end - addr) and guarantee old_len < new_len > > in mremap but I haven't checked that too deeply. > > Irrespective of that, just looking at the variables inside this > particular function where delta is an 'unsigned long', 'end' cannot > be less than vma->vm_end. Is not that true ?
no. What happens when end is too large? [...] > > here. This is hardly something that would save many cycles in a > > relatively cold path. > > Though I have not done any detailed instruction level measurement, > there is a reduction in real and system amount of time to execute > the test with and without the patch. > > Without the patch > > real 0m2.100s > user 0m0.162s > sys 0m1.937s > > With this patch > > real 0m0.928s > user 0m0.161s > sys 0m0.756s Are you telling me that two if conditions cause more than a second difference? That sounds suspicious. -- Michal Hocko SUSE Labs
