On Wed 02-03-22 12:26:45, Nico Pache wrote: > > > On 3/2/22 09:24, Michal Hocko wrote: > > Sorry, this has slipped through cracks. > > > > On Mon 14-02-22 15:39:31, Nico Pache wrote: > > [...] > >> We've recently been discussing the following if statement in __oom_reap_task_mm: > >> if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) > >> > >> Given the comment above it, and some of the upstream discussion the original > >> RFC, we are struggling to see why this should be a `||` and not an `&&`. If we > >> only want to reap anon memory and reaping shared memory can be dangerous is this > >> statement incorrect? > >> > >> We have a patch queued up to make this change, but wanted to get your opinion on > >> why this was originally designed this way in case we are missing something. > > > > I do not really see why this would be wrong. Private file backed > > mappings can contain a reapable memory as well. I do not see how this > > would solve the futex issue. > We were basing our discussion around the following comment: > /* > * Only anonymous pages have a good chance to be dropped > * without additional steps which we cannot afford as we > * are OOM already. > * > * We do not even care about fs backed pages because all > * which are reclaimable have already been reclaimed and > * we do not want to block exit_mmap by keeping mm ref > * count elevated without a good reason. > */ > > So changing to an && would align the functionality with this comment by ignoring > fs backed pages, and additionally it prevents shared mappings from being reaped. > We have tested this change and found we can no longer reproduce the issue. In > our case we allocate the mutex on a MAP_SHARED|MAP_ANONYMOUS mmap so the if- > statement in question would no longer return true after the && change. > > If it is the case that private fs backed pages matter perhaps we want something > like this: > if ((vma_is_anonymous(vma) && !(vma->vm_flags & VM_SHARED)) > ||(!vma_is_anonymous(vma) && !(vma->vm_flags & VM_SHARED))) > > or more simply: > if(!(vma->vm_flags & VM_SHARED)) > > to exclude all VM_SHARED mappings. I would have to think about that some more but I do not really see how this is related to the futex issue. In other words what kind of problem does this solve? -- Michal Hocko SUSE Labs
