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

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Jan 05, 2021 at 03:10:35PM +0800, Muchun Song wrote:
> On Tue, Jan 5, 2021 at 2:38 PM HORIGUCHI NAOYA(堀口　直也)
> <naoya.horiguchi@nec.com> wrote:
> >
> > On Mon, Jan 04, 2021 at 02:58:41PM +0800, Muchun Song wrote:
> > > When dissolve_free_huge_page() races with __free_huge_page(), we can
> > > do a retry. Because the race window is small.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > ---
> > >  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;
> > > +             }
> >
> > If dissolve_free_huge_page() races with __free_huge_page() and we detect
> > it with this check, the hugepage is expected to be freed or dissolved by
> > __free_huge_page(), so is it enough just to return with rc = 0 without retrying?
> 
> The dissolve_free_huge_page() aims to free the page to the buddy
> allocator not the hugepage pool. So it is not enough just to return 0,
> right? Or did you mean that we set the page temporary here and
> let the __free_huge_page do the freeing later for us? Thanks.

Ah, OK. You're right.
Thank you for the answer (and finding/fixing a few bugs).

- Naoya