Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when possible

[Yang Shi]

On Thu, Jun 16, 2022 at 7:27 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> On 2022/6/16 23:53, Yang Shi wrote:
> > On Thu, Jun 16, 2022 at 12:42 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> >>
> >> On 2022/6/16 7:58, Yang Shi wrote:
> >>> On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> >>>>
> >>>> Transhuge swapcaches won't be freed in __collapse_huge_page_copy().
> >>>> It's because release_pte_page() is not called for these pages and
> >>>> thus free_page_and_swap_cache can't grab the page lock. These pages
> >>>> won't be freed from swap cache even if we are the only user until
> >>>> next time reclaim. It shouldn't hurt indeed, but we could try to
> >>>> free these pages to save more memory for system.
> >>>
> >>>
> >>>>
> >>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> >>>> ---
> >>>>  include/linux/swap.h | 5 +++++
> >>>>  mm/khugepaged.c      | 1 +
> >>>>  mm/swap.h            | 5 -----
> >>>>  3 files changed, 6 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
> >>>> index 8672a7123ccd..ccb83b12b724 100644
> >>>> --- a/include/linux/swap.h
> >>>> +++ b/include/linux/swap.h
> >>>> @@ -456,6 +456,7 @@ static inline unsigned long total_swapcache_pages(void)
> >>>>         return global_node_page_state(NR_SWAPCACHE);
> >>>>  }
> >>>>
> >>>> +extern void free_swap_cache(struct page *page);
> >>>>  extern void free_page_and_swap_cache(struct page *);
> >>>>  extern void free_pages_and_swap_cache(struct page **, int);
> >>>>  /* linux/mm/swapfile.c */
> >>>> @@ -540,6 +541,10 @@ static inline void put_swap_device(struct swap_info_struct *si)
> >>>>  /* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
> >>>>  #define free_swap_and_cache(e) is_pfn_swap_entry(e)
> >>>>
> >>>> +static inline void free_swap_cache(struct page *page)
> >>>> +{
> >>>> +}
> >>>> +
> >>>>  static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
> >>>>  {
> >>>>         return 0;
> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >>>> index ee0a719c8be9..52109ad13f78 100644
> >>>> --- a/mm/khugepaged.c
> >>>> +++ b/mm/khugepaged.c
> >>>> @@ -756,6 +756,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> >>>>         list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
> >>>>                 list_del(&src_page->lru);
> >>>>                 release_pte_page(src_page);
> >>>> +               free_swap_cache(src_page);
> >>>
> >>> Will this really work? The free_swap_cache() will just dec refcounts
> >>> without putting the page back to buddy. So the hugepage is not
> >>> actually freed at all. Am I missing something?
> >>
> >> Thanks for catching this! If page is on percpu lru_pvecs cache, page will
> >> be released when lru_pvecs are drained. But if not, free_swap_cache() won't
> >> free the page as it assumes the caller has a reference on the page and thus
> >> only does page_ref_sub(). Does the below change looks sense for you?
> >
> > THP gets drained immediately so they won't stay in pagevecs.
>
> Yes, you're right. I missed this.
>
> >
> >>
> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >> index 52109ad13f78..b8c96e33591d 100644
> >> --- a/mm/khugepaged.c
> >> +++ b/mm/khugepaged.c
> >> @@ -755,8 +755,12 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> >>
> >>         list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
> >>                 list_del(&src_page->lru);
> >> -               release_pte_page(src_page);
> >> +               mod_node_page_state(page_pgdat(src_page),
> >> +                                   NR_ISOLATED_ANON + page_is_file_lru(src_page),
> >> +                                   -compound_nr(src_page));
> >> +               unlock_page(src_page);
> >>                 free_swap_cache(src_page);
> >> +               putback_lru_page(src_page);
> >
> > I'm not sure if it is worth it or not for a rare corner case since THP
> > should not stay in swapcache unless try_to_unmap() in vmscan fails
>
> IIUC, even if try_to_unmap() in vmscan succeeds, THP might be still in the
> swapcache if shrink_page_list is not called for this THP again after writeback
> is done, e.g. when shrink_page_list is called from madvise, so there might be

I don't get, doesn't __remove_mapping() delete the page from swap cache?

> no memory pressure, or do_swap_page puts the THP into page table again. Also THP

do_swap_page() just swaps in base page, never THP.

> might not be splited when deferred_split_shrinker is not called, e.g. due to

I don't see how deferred split is related to this.

> not lacking of memory. Even if there is memory pressure, the THP will stay in
> swapcache until next round page reclaim for this THP is done. So there should
> be a non-negligible window that THP will stay in the swapcache.
> Or am I miss something?

I guess you may misunderstand what I meant. This patch is trying to
optimize freeing THP in swapcache. But it should be very rare that
khugepaged sees THP from swap cache. The only case I could think of is
try_to_unmap() in vmscan fails. That might leave THP in swap cache so
that khugepaged could see it.


>
> > IIUC. And it is not guaranteed that free_swap_cache() will get the
> > page lock.
>
> IMHO, we're not guaranteed that free_swap_cache() will get the page lock for the normal
> page anyway.
>
> Thanks!
>
> >
> >>         }
> >>  }
> >>
> >> Thanks!
> >>
> >>>
> >>>>         }
> >>>>  }
> >>>>
> >>>> diff --git a/mm/swap.h b/mm/swap.h
> >>>> index 0193797b0c92..863f6086c916 100644
> >>>> --- a/mm/swap.h
> >>>> +++ b/mm/swap.h
> >>>> @@ -41,7 +41,6 @@ void __delete_from_swap_cache(struct page *page,
> >>>>  void delete_from_swap_cache(struct page *page);
> >>>>  void clear_shadow_from_swap_cache(int type, unsigned long begin,
> >>>>                                   unsigned long end);
> >>>> -void free_swap_cache(struct page *page);
> >>>>  struct page *lookup_swap_cache(swp_entry_t entry,
> >>>>                                struct vm_area_struct *vma,
> >>>>                                unsigned long addr);
> >>>> @@ -81,10 +80,6 @@ static inline struct address_space *swap_address_space(swp_entry_t entry)
> >>>>         return NULL;
> >>>>  }
> >>>>
> >>>> -static inline void free_swap_cache(struct page *page)
> >>>> -{
> >>>> -}
> >>>> -
> >>>>  static inline void show_swap_cache_info(void)
> >>>>  {
> >>>>  }
> >>>> --
> >>>> 2.23.0
> >>>>
> >>>>
> >>> .
> >>>
> >>
> > .
> >
>