On Fri, 25 May 2018, Andrew Morton wrote:
> > > > --- a/mm/hugetlb.c
> > > > +++ b/mm/hugetlb.c
> > > > @@ -2006,8 +2006,10 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
> > > > * code of zero indicates a reservation exists (no change).
> > > > */
> > > > map_chg = gbl_chg = vma_needs_reservation(h, vma, addr);
> > > > - if (map_chg < 0)
> > > > - return ERR_PTR(-ENOMEM);
> > > > + if (map_chg < 0) {
> > > > + ret = -ENOMEM;
> > > > + goto out;
> > > > + }
> > >
> > > This doesn't change the return value.
> > >
> >
> > This, and the subsequent comments, are referring to the third paragraph of
> > the changelog.
> >
> > The functional part of the change is for the
> > hugetlb_cgroup_charge_cgroup() return value that is now actually used.
>
>
> Ah. Missed that bit.
>
If you'd like this separated into two separate patches, one that fixes the
actual issue with the hugetlb_cgroup_charge_cgroup() return value and the
other to use a single exit path with ERR_PTR(ret), that might make it
easier. I think the latter is why the bug was introduced: it's too easy
to force -ENOSPC unintentionally.
> Altered changelog:
>
> : When charging to a hugetlb_cgroup fails, alloc_huge_page() returns
> : ERR_PTR(-ENOSPC) which will cause VM_FAULT_SIGBUS to be returned to the
> : page fault handler.
> :
> : This is because the return value from hugetlb_cgroup_charge_cgroup() is
> : overwritten with ERR_PTR(-ENOSPC).
> :
> : Instead, propagate the error code from hugetlb_cgroup_charge_cgroup()
> : (ERR_PTR(-ENOMEM)), so VM_FAULT_OOM is handled correctly. This is
> : consistent with failing mem cgroup charges in the non-hugetlb fault path.
> :
> : At the same time, restructure the return paths of alloc_huge_page() so it
> : is consistent.
>
LGTM, thanks.
> >
> > > It would be nice if you could add a comment over alloc_huge_page()
> > > explaining the return values (at least). Why sometimes ENOMEM, other
> > > times ENOSPC?
> > >
> >
> > The ENOSPC is used to specifically induce a VM_FAULT_SIGBUS, which
> > Documentation/vm/hugetlbfs_reserv.txt specifies is how faults are handled
> > if no hugetlb pages are available.
>
> That wasn't a code comment ;) Nobody will know to go looking in
> hugetlbfs_reserv.txt - it isn't even referred to from mm/*.c.
>
Let's see what Mike and Aneesh say, because they may object to using
VM_FAULT_OOM because there's no way to guarantee that we'll come under the
limit of hugetlb_cgroup as a result of the oom. My assumption is that we
use VM_FAULT_SIGBUS since oom killing will not guarantee that the
allocation can succeed. But now a process can get a SIGBUS if its hugetlb
pages are not allocatable or its under a limit imposed by hugetlb_cgroup
that it's not aware of. Faulting hugetlb pages is certainly risky
business these days...
Perhaps the optimal solution for reaching hugetlb_cgroup limits is to
induce an oom kill from within the hugetlb_cgroup itself? Otherwise the
unlucky process to fault their hugetlb pages last gets SIGBUS.
