Re: [External] Re: [PATCH 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

[Muchun Song]

On Tue, Jan 5, 2021 at 11:14 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jan 5, 2021 at 9:33 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> >
> > On 1/3/21 10:58 PM, Muchun Song wrote:
> > > When dissolve_free_huge_page() races with __free_huge_page(), we can
> > > do a retry. Because the race window is small.
> >
> > In general, I agree that the race window is small.  However, worst case
> > would be if the freeing of the page is put on a work queue.  Is it acceptable
> > to keep retrying in that case?  In addition, the 'Free some vmemmap' series
> > may slow the free_huge_page path even more.
>
> I also consider the 'Free some vmemmap' series case. In my next
> version series, I will flush the work before dissolve_free_huge_page
> returns when encountering this race. So the retry is acceptable.
> Right?

Hi Mike,

How about flushing the @free_hpage_work when
encountering this race?

>
> Thanks.
>
> >
> > In these worst case scenarios, I am not sure we want to just spin retrying.
> >
> > --
> > Mike Kravetz
> >
> > >
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > ---
> > >  mm/hugetlb.c | 16 +++++++++++-----
> > >  1 file changed, 11 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 72608008f8b4..db00ae375d2a 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -1763,10 +1763,11 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > >   * nothing for in-use hugepages and non-hugepages.
> > >   * This function returns values like below:
> > >   *
> > > - *  -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > > - *          (allocated or reserved.)
> > > - *       0: successfully dissolved free hugepages or the page is not a
> > > - *          hugepage (considered as already dissolved)
> > > + *  -EAGAIN: race with __free_huge_page() and can do a retry
> > > + *  -EBUSY:  failed to dissolved free hugepages or the hugepage is in-use
> > > + *           (allocated or reserved.)
> > > + *       0:  successfully dissolved free hugepages or the page is not a
> > > + *           hugepage (considered as already dissolved)
> > >   */
> > >  int dissolve_free_huge_page(struct page *page)
> > >  {
> > > @@ -1815,8 +1816,10 @@ int dissolve_free_huge_page(struct page *page)
> > >                * We should make sure that the page is already on the free list
> > >                * when it is dissolved.
> > >                */
> > > -             if (unlikely(!PageHugeFreed(head)))
> > > +             if (unlikely(!PageHugeFreed(head))) {
> > > +                     rc = -EAGAIN;
> > >                       goto out;
> > > +             }
> > >
> > >               /*
> > >                * Move PageHWPoison flag from head page to the raw error page,
> > > @@ -1857,7 +1860,10 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> > >
> > >       for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
> > >               page = pfn_to_page(pfn);
> > > +retry:
> > >               rc = dissolve_free_huge_page(page);
> > > +             if (rc == -EAGAIN)
> > > +                     goto retry;
> > >               if (rc)
> > >                       break;
> > >       }
> > >