On Fri, 25 May 2018, Andrew Morton wrote:
> On Fri, 25 May 2018 13:16:45 -0700 (PDT) David Rientjes <[email protected]>
> wrote:
>
> > 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.
> >
> > Instead, return the proper error code, 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.
>
> Patch doesn't appear to match the changelog?
>
In what way?
> > --- 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.
> > /*
> > * Processes that did not create the mapping will have no
> > @@ -2019,8 +2021,8 @@ struct page *alloc_huge_page(struct vm_area_struct
> > *vma,
> > if (map_chg || avoid_reserve) {
> > gbl_chg = hugepage_subpool_get_pages(spool, 1);
> > if (gbl_chg < 0) {
> > - vma_end_reservation(h, vma, addr);
> > - return ERR_PTR(-ENOSPC);
> > + ret = -ENOSPC;
> > + goto out_reservation;
> > }
>
> Nor does this.
>
> > /*
> > @@ -2049,8 +2051,10 @@ struct page *alloc_huge_page(struct vm_area_struct
> > *vma,
> > if (!page) {
> > spin_unlock(&hugetlb_lock);
> > page = alloc_buddy_huge_page_with_mpol(h, vma, addr);
> > - if (!page)
> > + if (!page) {
> > + ret = -ENOSPC;
> > goto out_uncharge_cgroup;
> > + }
>
> Nor does this.
>
> > if (!avoid_reserve && vma_has_reserves(vma, gbl_chg)) {
> > SetPagePrivate(page);
> > h->resv_huge_pages--;
> > @@ -2087,8 +2091,10 @@ struct page *alloc_huge_page(struct vm_area_struct
> > *vma,
> > out_subpool_put:
> > if (map_chg || avoid_reserve)
> > hugepage_subpool_put_pages(spool, 1);
> > +out_reservation:
> > vma_end_reservation(h, vma, addr);
> > - return ERR_PTR(-ENOSPC);
> > +out:
> > + return ERR_PTR(ret);
> > }
> >
>
> 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.
