Re: [PATCH v2 1/2] hugetlb: Do not clear hugetlb dtor until allocating vmemmap

[Jiaqi Yan <jiaqiyan@google.com>]

On Mon, Jul 17, 2023 at 5:50 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> Freeing a hugetlb page and releasing base pages back to the underlying
> allocator such as buddy or cma is performed in two steps:
> - remove_hugetlb_folio() is called to remove the folio from hugetlb
>   lists, get a ref on the page and remove hugetlb destructor.  This
>   all must be done under the hugetlb lock.  After this call, the page
>   can be treated as a normal compound page or a collection of base
>   size pages.
> - update_and_free_hugetlb_folio() is called to allocate vmemmap if
>   needed and the free routine of the underlying allocator is called
>   on the resulting page.  We can not hold the hugetlb lock here.
>
> One issue with this scheme is that a memory error could occur between
> these two steps.  In this case, the memory error handling code treats
> the old hugetlb page as a normal compound page or collection of base
> pages.  It will then try to SetPageHWPoison(page) on the page with an
> error.  If the page with error is a tail page without vmemmap, a write
> error will occur when trying to set the flag.
>
> Address this issue by modifying remove_hugetlb_folio() and
> update_and_free_hugetlb_folio() such that the hugetlb destructor is not
> cleared until after allocating vmemmap.  Since clearing the destructor
> requires holding the hugetlb lock, the clearing is done in
> remove_hugetlb_folio() if the vmemmap is present.  This saves a
> lock/unlock cycle.  Otherwise, destructor is cleared in
> update_and_free_hugetlb_folio() after allocating vmemmap.
>
> Note that this will leave hugetlb pages in a state where they are marked
> free (by hugetlb specific page flag) and have a ref count.  This is not
> a normal state.  The only code that would notice is the memory error
> code, and it is set up to retry in such a case.
>
> A subsequent patch will create a routine to do bulk processing of
> vmemmap allocation.  This will eliminate a lock/unlock cycle for each
> hugetlb page in the case where we are freeing a large number of pages.
>
> Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  mm/hugetlb.c | 90 ++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 66 insertions(+), 24 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 64a3239b6407..4a910121a647 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1579,9 +1579,37 @@ static inline void destroy_compound_gigantic_folio(struct folio *folio,
>                                                 unsigned int order) { }
>  #endif
>
> +static inline void __clear_hugetlb_destructor(struct hstate *h,
> +                                               struct folio *folio)
> +{
> +       lockdep_assert_held(&hugetlb_lock);
> +
> +       /*
> +        * Very subtle
> +        *
> +        * For non-gigantic pages set the destructor to the normal compound
> +        * page dtor.  This is needed in case someone takes an additional
> +        * temporary ref to the page, and freeing is delayed until they drop
> +        * their reference.
> +        *
> +        * For gigantic pages set the destructor to the null dtor.  This
> +        * destructor will never be called.  Before freeing the gigantic
> +        * page destroy_compound_gigantic_folio will turn the folio into a
> +        * simple group of pages.  After this the destructor does not
> +        * apply.
> +        *
> +        */
> +       if (hstate_is_gigantic(h))
> +               folio_set_compound_dtor(folio, NULL_COMPOUND_DTOR);
> +       else
> +               folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
> +}
> +
>  /*
> - * Remove hugetlb folio from lists, and update dtor so that the folio appears
> - * as just a compound page.
> + * Remove hugetlb folio from lists.
> + * If vmemmap exists for the folio, update dtor so that the folio appears
> + * as just a compound page.  Otherwise, wait until after allocating vmemmap
> + * to update dtor.
>   *
>   * A reference is held on the folio, except in the case of demote.
>   *
> @@ -1612,31 +1640,19 @@ static void __remove_hugetlb_folio(struct hstate *h, struct folio *folio,
>         }
>
>         /*
> -        * Very subtle
> -        *
> -        * For non-gigantic pages set the destructor to the normal compound
> -        * page dtor.  This is needed in case someone takes an additional
> -        * temporary ref to the page, and freeing is delayed until they drop
> -        * their reference.
> -        *
> -        * For gigantic pages set the destructor to the null dtor.  This
> -        * destructor will never be called.  Before freeing the gigantic
> -        * page destroy_compound_gigantic_folio will turn the folio into a
> -        * simple group of pages.  After this the destructor does not
> -        * apply.
> -        *
> -        * This handles the case where more than one ref is held when and
> -        * after update_and_free_hugetlb_folio is called.
> -        *
> -        * In the case of demote we do not ref count the page as it will soon
> -        * be turned into a page of smaller size.
> +        * We can only clear the hugetlb destructor after allocating vmemmap
> +        * pages.  Otherwise, someone (memory error handling) may try to write
> +        * to tail struct pages.
> +        */
> +       if (!folio_test_hugetlb_vmemmap_optimized(folio))
> +               __clear_hugetlb_destructor(h, folio);
> +
> +        /*
> +         * In the case of demote we do not ref count the page as it will soon
> +         * be turned into a page of smaller size.
>          */
>         if (!demote)
>                 folio_ref_unfreeze(folio, 1);
> -       if (hstate_is_gigantic(h))
> -               folio_set_compound_dtor(folio, NULL_COMPOUND_DTOR);
> -       else
> -               folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
>
>         h->nr_huge_pages--;
>         h->nr_huge_pages_node[nid]--;
> @@ -1728,6 +1744,19 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
>                 return;
>         }
>
> +       /*
> +        * If needed, clear hugetlb destructor under the hugetlb lock.
> +        * This must be done AFTER allocating vmemmap pages in case there is an
> +        * attempt to write to tail struct pages as in memory poison.
> +        * It must be done BEFORE PageHWPoison handling so that any subsequent
> +        * memory errors poison individual pages instead of head.
> +        */
> +       if (folio_test_hugetlb(folio)) {

Thanks Mike, this definitely fixed the issue in v1, :)

> +               spin_lock_irq(&hugetlb_lock);
> +               __clear_hugetlb_destructor(h, folio);
> +               spin_unlock_irq(&hugetlb_lock);
> +       }
> +
>         /*
>          * Move PageHWPoison flag from head page to the raw error pages,
>          * which makes any healthy subpages reusable.
> @@ -3604,6 +3633,19 @@ static int demote_free_hugetlb_folio(struct hstate *h, struct folio *folio)
>                 return rc;
>         }
>
> +       /*
> +        * The hugetlb destructor could still be set for this folio if vmemmap
> +        * was actually allocated above.  The ref count on all pages is 0.
> +        * Therefore, nobody should attempt access.  However, before destroying
> +        * compound page below, clear the destructor. Unfortunately, this
> +        * requires a lock/unlock cycle.
> +        */
> +       if (folio_test_hugetlb(folio)) {
> +               spin_lock_irq(&hugetlb_lock);
> +               __clear_hugetlb_destructor(h, folio);
> +               spin_unlock_irq(&hugetlb_lock);
> +       }
> +
>         /*
>          * Use destroy_compound_hugetlb_folio_for_demote for all huge page
>          * sizes as it will not ref count folios.
> --
> 2.41.0
>