On Thu, 15 Jun 2017, Michal Hocko wrote: > > If mm->mm_users is not incremented because it is already zero by the oom > > reaper, meaning the final refcount has been dropped, do not set > > MMF_OOM_SKIP prematurely. > > > > __mmput() may not have had a chance to do exit_mmap() yet, so memory from > > a previous oom victim is still mapped. > > true and do we have a _guarantee_ it will do it? E.g. can somebody block > exit_aio from completing? Or can somebody hold mmap_sem and thus block > ksm_exit resp. khugepaged_exit from completing? The reason why I was > conservative and set such a mm as MMF_OOM_SKIP was because I couldn't > give a definitive answer to those questions. And we really _want_ to > have a guarantee of a forward progress here. Killing an additional > proecess is a price to pay and if that doesn't trigger normall it sounds > like a reasonable compromise to me. > I have not seen any issues where __mmput() stalls and exit_mmap() fails to free its mapped memory once mm->mm_users has dropped to 0. > > __mput() naturally requires no > > references on mm->mm_users to do exit_mmap(). > > > > Without this, several processes can be oom killed unnecessarily and the > > oom log can show an abundance of memory available if exit_mmap() is in > > progress at the time the process is skipped. > > Have you seen this happening in the real life? > Yes, quite a bit in testing. One oom kill shows the system to be oom: [22999.488705] Node 0 Normal free:90484kB min:90500kB ... [22999.488711] Node 1 Normal free:91536kB min:91948kB ... followed up by one or more unnecessary oom kills showing the oom killer racing with memory freeing of the victim: [22999.510329] Node 0 Normal free:229588kB min:90500kB ... [22999.510334] Node 1 Normal free:600036kB min:91948kB ... The patch is absolutely required for us to prevent continuous oom killing of processes after a single process has been oom killed and its memory is in the process of being freed. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
