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Message-ID: <CAHbLzkqAJK3C8svOm4mhCP_oP6od=NMKpeW9=oRcczDJ=FXPcw@mail.gmail.com>
Date: Thu, 16 Jun 2022 08:53:29 -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 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.
>
> 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. And it is not guaranteed that free_swap_cache() will get the
page lock.
> }
> }
>
> 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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