Re: [PATCH v2] mm/khugepaged: alloc_charge_hpage() take care of mem charge errors

[Peter Xu <peterx@redhat.com>]

On Thu, Mar 02, 2023 at 03:21:50PM -0800, Zach O'Keefe wrote:
> On Feb 22 14:53, Yang Shi wrote:
> > On Wed, Feb 22, 2023 at 11:52 AM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > If memory charge failed, instead of returning the hpage but with an error,
> > > allow the function to cleanup the folio properly, which is normally what a
> > > function should do in this case - either return successfully, or return
> > > with no side effect of partial runs with an indicated error.
> > >
> > > This will also avoid the caller calling mem_cgroup_uncharge() unnecessarily
> > > with either anon or shmem path (even if it's safe to do so).
> > 
> > Thanks for the cleanup. Reviewed-by: Yang Shi <shy828301@gmail.com>
> > 
> > >
> > > Cc: Yang Shi <shy828301@gmail.com>
> > > Reviewed-by: David Stevens <stevensd@chromium.org>
> > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > > v1->v2:
> > > - Enhance commit message, drop "Fixes:" and "Cc: stable" tag, add R-bs.
> > > ---
> > >  mm/khugepaged.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 8dbc39896811..941d1c7ea910 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1063,12 +1063,19 @@ static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
> > >         gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
> > >                      GFP_TRANSHUGE);
> > >         int node = hpage_collapse_find_target_node(cc);
> > > +       struct folio *folio;
> > >
> > >         if (!hpage_collapse_alloc_page(hpage, gfp, node, &cc->alloc_nmask))
> > >                 return SCAN_ALLOC_HUGE_PAGE_FAIL;
> > > -       if (unlikely(mem_cgroup_charge(page_folio(*hpage), mm, gfp)))
> > > +
> > > +       folio = page_folio(*hpage);
> > > +       if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
> > > +               folio_put(folio);
> > > +               *hpage = NULL;
> > >                 return SCAN_CGROUP_CHARGE_FAIL;
> > > +       }
> > >         count_memcg_page_event(*hpage, THP_COLLAPSE_ALLOC);
> > > +
> > >         return SCAN_SUCCEED;
> > >  }
> > >
> > > --
> > > 2.39.1
> > >
> > 
> 
> Thanks, Peter.
> 
> Can we also get rid of the unnecessary mem_cgroup_uncharge() calls while we're
> at it? Maybe this deserves a separate patch, but after Yang's cleanup of the
> !NUMA case (where we would preallocate a hugepage) we can depend on put_page()
> do take care of that for us.

Makes sense to me.  I can prepare a separate patch to clean it up.

> 
> Regardless, can have my
> 
> Reviewed-by: Zach O'Keefe <zokeefe@google.com>

Thanks!

-- 
Peter Xu