Re: [PATCH v2 07/11] hugetlb: perform vmemmap restoration on a list of pages

[Mike Kravetz]

On 09/07/23 11:54, Mike Kravetz wrote:
> On 09/07/23 11:33, Muchun Song wrote:
> > 
> > 
> > > On Sep 7, 2023, at 05:12, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > > 
> > > On 09/06/23 16:07, Muchun Song wrote:
> > >>> On Sep 6, 2023, at 15:33, Muchun Song <muchun.song@xxxxxxxxx> wrote:
> > >>> On 2023/9/6 05:44, Mike Kravetz wrote:
> > >>>> When removing hugetlb pages from the pool, we first create a list
> > >>>> of removed pages and then free those pages back to low level allocators.
> > >>>> Part of the 'freeing process' is to restore vmemmap for all base pages
> > >>>> if necessary.  Pass this list of pages to a new routine
> > >>>> hugetlb_vmemmap_restore_folios() so that vmemmap restoration can be
> > >>>> performed in bulk.
> > >>>> 
> > >>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > >>>> ---
> > >>>> mm/hugetlb.c         |  3 +++
> > >>>> mm/hugetlb_vmemmap.c | 13 +++++++++++++
> > >>>> mm/hugetlb_vmemmap.h |  5 +++++
> > >>>> 3 files changed, 21 insertions(+)
> > >>>> 
> > >>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > >>>> index 554be94b07bd..dd2dbc256172 100644
> > >>>> --- a/mm/hugetlb.c
> > >>>> +++ b/mm/hugetlb.c
> > >>>> @@ -1838,6 +1838,9 @@ static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
> > >>>> {
> > >>>>  struct folio *folio, *t_folio;
> > >>>> + /* First restore vmemmap for all pages on list. */
> > >>>> + hugetlb_vmemmap_restore_folios(h, list);
> > >>>> +
> > >>>>  list_for_each_entry_safe(folio, t_folio, list, lru) {
> > >>>>  update_and_free_hugetlb_folio(h, folio, false);
> > >>>>  cond_resched();
> > >>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > >>>> index ac5577d372fe..79de984919ef 100644
> > >>>> --- a/mm/hugetlb_vmemmap.c
> > >>>> +++ b/mm/hugetlb_vmemmap.c
> > >>>> @@ -481,6 +481,19 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
> > >>>>  return ret;
> > >>>> }
> > >>>> +/*
> > >>>> + * This function will attempt to resore vmemmap for a list of folios.  There
> > >>>> + * is no guarantee that restoration will be successful for all or any folios.
> > >>>> + * This is used in bulk operations, and no feedback is given to the caller.
> > >>>> + */
> > >>>> +void hugetlb_vmemmap_restore_folios(const struct hstate *h, struct list_head *folio_list)
> > >>>> +{
> > >>>> + struct folio *folio;
> > >>>> +
> > >>>> + list_for_each_entry(folio, folio_list, lru)
> > >>>> + (void)hugetlb_vmemmap_restore(h, &folio->page);
> > >>> 
> > >>> I am curious about the purpose of "void" here, seems it it not necessnary,
> > >>> ritgh? We cound see so many palces where we do not add the void if the caller
> > >>> does not care about the return value of the callee.
> > >> 
> > >> Another question: should we stop restoring vmemmap pages when
> > >> hugetlb_vmemmap_restore() fails? In which case, I suspect there
> > >> is no memory probably, there is no need to continue, right?
> > > 
> > > Recall that the list of hugetlb pages may be from multiple nodes.  My first
> > > thought was that we should continue because memory allocation may fail on one
> > > node but succeed on another.  However, with
> > > https://lore.kernel.org/linux-mm/20230905031312.91929-1-yuancan@xxxxxxxxxx/
> > > memory allocation should fall back to other nodes.  So, yes I do believe it
> > > would make sense to stop when hugetlb_vmemmap_restore returns ENOMEM as
> > > we are unlikely to make forward progress.
> > 
> > Agree.
> > 
> > > 
> > > Today's behavior will try to restore vmemmap for all pages.  No stopping
> > > on error.
> > > 
> > > I have mixed thoughts on this.  Quitting on error 'seems reasonable'.
> > > However, if we continue we 'might' be able to allocate vmemmap for one
> > > hugetlb page.  And, if we free one hugetlb page that should provide
> > > vmemmap for several more and we may be able to free most pages on the
> > > list.
> > 
> > Yes. A good point. But there should be a non-optimized huge page been
> > freed somewhere in parallel, otherwise we still cannot allocate memory.
> 
> It does not have to be another huge page being freed in parallel.  It
> could be that when allocating vmemmap for a 1G hugetlb page we were one
> (4K) page short of what was required.  If someone else frees a 4K page,
> freeing the next 1G page may succeed.
> -- 
> Mike Kravetz
> 
> > However, the freeing operation happens after hugetlb_vmemmap_restore_folios.
> > If we want to handle this, we should rework update_and_free_pages_bulk()
> > to do a try when at least a huge pages is freed.

This seemed familiar.  Recall this patch which Muchun Reviewed and James Acked,
https://lore.kernel.org/linux-mm/20230718004942.113174-3-mike.kravetz@xxxxxxxxxx/

If we can not restore vmemmap for a page, then it must be turned into a
surplus huge page.  In this patch (not the previous one referenced), we
will try to restore vmemmap one more time in a later call to
update_and_free_hugetlb_folio.  Certainly, we do not want to try twice!

My 'plan' is to include the previous patch as part of this series.  With
that patch in place, the list_for_each_entry calls to hugetlb_vmemmap_restore
can be replaced with a call to hugetlb_vmemmap_restore_folios.  We would
change the behavior of hugetlb_vmemmap_restore_folios to return an error
instead of being of type void.  If an error is returned, then we will
make another pass through the list looking for unoptimized pages and add
them as surplus.

I think it best if we try to restore vmemmap at least once before
converting to a surplus page.
-- 
Mike Kravetz