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

[Mike Kravetz <mike.kravetz@oracle.com>]

On 09/06/23 16:07, Muchun Song wrote:
> > On Sep 6, 2023, at 15:33, Muchun Song <muchun.song@linux.dev> 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@oracle.com>
> >> ---
> >>  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@huawei.com/
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.

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.
-- 
Mike Kravetz