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Message-ID: <CAHbLzkq+-XeFJYry8j80VUDbY6BY2dFNA5FT3u9eF-v5MckQTA@mail.gmail.com>
Date: Fri, 17 Jun 2022 09:35:58 -0700
From: Yang Shi <shy828301@...il.com>
To: Miaohe Lin <linmiaohe@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>,
David Howells <dhowells@...hat.com>, NeilBrown <neilb@...e.de>,
Alistair Popple <apopple@...dia.com>,
David Hildenbrand <david@...hat.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Peter Xu <peterx@...hat.com>, Linux MM <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when possible
On Thu, Jun 16, 2022 at 7:27 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
>
> On 2022/6/16 23:53, Yang Shi wrote:
> > On Thu, Jun 16, 2022 at 12:42 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
> >>
> >> On 2022/6/16 7:58, Yang Shi wrote:
> >>> On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@...wei.com> 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@...wei.com>
> >>>> ---
> >>>> 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
> >>>>
> >>>>
> >>> .
> >>>
> >>
> > .
> >
>
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