On 07/06/22 11:04, Miaohe Lin wrote:
> On 2022/7/5 14:39, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Tue, Jul 05, 2022 at 10:16:39AM +0800, Miaohe Lin wrote:
> >> On 2022/7/4 9:33, Naoya Horiguchi wrote:
> >>> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> >>>
> >>> I found a weird state of 1GB hugepage pool, caused by the following
> >>> procedure:
> >>>
> >>> - run a process reserving all free 1GB hugepages,
> >>> - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
> >>> /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
> >>> - kill the reserving process.
> >>>
> >>> , then all the hugepages are free *and* surplus at the same time.
> >>>
> >>> $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> >>> 3
> >>> $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
> >>> 3
> >>> $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
> >>> 0
> >>> $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
> >>> 3
> >>>
> >>> This state is resolved by reserving and allocating the pages then
> >>> freeing them again, so this seems not to result in serious problem.
> >>> But it's a little surprising (shrinking pool suddenly fails).
> >>>
> >>> This behavior is caused by hstate_is_gigantic() check in
> >>> return_unused_surplus_pages(). This was introduced so long ago in 2008
> >>> by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> >>> at that time the gigantic pages were not supposed to be allocated/freed
> >>> at run-time. Now kernel can support runtime allocation/free, so let's
> >>> check gigantic_page_runtime_supported() together.
> >>>
> >>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> >>
> >> This patch looks good to me with a few question below.
> >
> > Thank you for reviewing.
> >
> >>
> >>> ---
> >>> v2 -> v3:
> >>> - Fixed typo in patch description,
> >>> - add !gigantic_page_runtime_supported() check instead of removing
> >>> hstate_is_gigantic() check (suggested by Miaohe and Muchun)
> >>> - add a few more !gigantic_page_runtime_supported() check in
> >>> set_max_huge_pages() (by Mike).
> >>> ---
> >>> mm/hugetlb.c | 19 ++++++++++++++++---
> >>> 1 file changed, 16 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >>> index 2a554f006255..bdc4499f324b 100644
> >>> --- a/mm/hugetlb.c
> >>> +++ b/mm/hugetlb.c
> >>> @@ -2432,8 +2432,7 @@ static void return_unused_surplus_pages(struct hstate *h,
> >>> /* Uncommit the reservation */
> >>> h->resv_huge_pages -= unused_resv_pages;
> >>>
> >>> - /* Cannot return gigantic pages currently */
> >>> - if (hstate_is_gigantic(h))
> >>> + if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >>> goto out;
> >>>
> >>> /*
> >>> @@ -3315,7 +3314,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
> >>> * the user tries to allocate gigantic pages but let the user free the
> >>> * boottime allocated gigantic pages.
> >>> */
> >>> - if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
> >>> + if (hstate_is_gigantic(h) && (!IS_ENABLED(CONFIG_CONTIG_ALLOC) ||
> >>> + !gigantic_page_runtime_supported())) {
> >>> if (count > persistent_huge_pages(h)) {
> >>> spin_unlock_irq(&hugetlb_lock);
> >>> mutex_unlock(&h->resize_lock);
> >>> @@ -3363,6 +3363,19 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
> >>> goto out;
> >>> }
> >>>
> >>> + /*
> >>> + * We can not decrease gigantic pool size if runtime modification
> >>> + * is not supported.
> >>> + */
> >>> + if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) {
> >>> + if (count < persistent_huge_pages(h)) {
> >>> + spin_unlock_irq(&hugetlb_lock);
> >>> + mutex_unlock(&h->resize_lock);
> >>> + NODEMASK_FREE(node_alloc_noretry);
> >>> + return -EINVAL;
> >>> + }
> >>> + }
> >>
> >> With above change, we're not allowed to decrease the pool size now. But it was allowed previously
> >> even if !gigantic_page_runtime_supported. Does this will break user?
> >
> > Yes, it does. I might get the wrong idea about the definition of
> > gigantic_page_runtime_supported(), which shows that runtime pool *extension*
> > is supported or not (implying that pool shrinking is always possible).
> > If this is right, this new if-block is not necessary.
>
> I tend to remove above new if-block to keep pool shrinking available.
>
Not sure I am following the questions.
Take a look at __update_and_free_page which will refuse to 'free' a
gigantic page if !gigantic_page_runtime_supported. I 'think' attempting
to shrink the pool when !gigantic_page_runtime_supported will result in
leaking gigantic pages. i.e. Memory will remain allocated for the
gigantic page, but it can not be used.
I can take a closer look during my tomorrow.
IIRC, the only way gigantic_page_runtime_supported is not set to day is
in the case of powerpc using 16GB pages allocated/managed by firmware.
--
Mike Kravetz
