On Fri, 25 May 2018 13:59:40 -0700 (PDT) David Rientjes <[email protected]>
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.
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.
>
> > 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.
