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Message-ID: <CANzGp4J_7-Rj8N3jM3Tb+zAMZm21nf-cni0RjWWWfJNDXu5asQ@mail.gmail.com>
Date: Fri, 12 Apr 2024 19:38:55 +0800
From: Chuanhua Han <chuanhuahan@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org, linux-mm@...ck.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com,
hanchuanhua@...o.com, hannes@...xchg.org, hughd@...gle.com,
kasong@...cent.com, surenb@...gle.com, v-songbaohua@...o.com,
willy@...radead.org, xiang@...nel.org, ying.huang@...el.com,
yosryahmed@...gle.com, yuzhao@...gle.com, ziy@...dia.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] mm: swap: introduce swap_free_nr() for batched swap_free()
Ryan Roberts <ryan.roberts@....com> 于2024年4月12日周五 19:28写道：
>
> On 12/04/2024 03:07, Chuanhua Han wrote:
> > Ryan Roberts <ryan.roberts@....com> 于2024年4月11日周四 22:30写道：
> >>
> >> On 09/04/2024 09:26, Barry Song wrote:
> >>> From: Chuanhua Han <hanchuanhua@...o.com>
> >>>
> >>> While swapping in a large folio, we need to free swaps related to the whole
> >>> folio. To avoid frequently acquiring and releasing swap locks, it is better
> >>> to introduce an API for batched free.
> >>>
> >>> Signed-off-by: Chuanhua Han <hanchuanhua@...o.com>
> >>> Co-developed-by: Barry Song <v-songbaohua@...o.com>
> >>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> >>
> >> Couple of nits; feel free to ignore.
> >>
> >> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> >>
> >>> ---
> >>> include/linux/swap.h | 5 +++++
> >>> mm/swapfile.c | 51 ++++++++++++++++++++++++++++++++++++++++++++
> >>> 2 files changed, 56 insertions(+)
> >>>
> >>> diff --git a/include/linux/swap.h b/include/linux/swap.h
> >>> index 11c53692f65f..b7a107e983b8 100644
> >>> --- a/include/linux/swap.h
> >>> +++ b/include/linux/swap.h
> >>> @@ -483,6 +483,7 @@ extern void swap_shmem_alloc(swp_entry_t);
> >>> extern int swap_duplicate(swp_entry_t);
> >>> extern int swapcache_prepare(swp_entry_t);
> >>> extern void swap_free(swp_entry_t);
> >>> +extern void swap_free_nr(swp_entry_t entry, int nr_pages);
> >>> extern void swapcache_free_entries(swp_entry_t *entries, int n);
> >>> extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
> >>> int swap_type_of(dev_t device, sector_t offset);
> >>> @@ -564,6 +565,10 @@ static inline void swap_free(swp_entry_t swp)
> >>> {
> >>> }
> >>>
> >>> +void swap_free_nr(swp_entry_t entry, int nr_pages)
> >>> +{
> >>> +}
> >>> +
> >>> static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
> >>> {
> >>> }
> >>> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >>> index 28642c188c93..f4c65aeb088d 100644
> >>> --- a/mm/swapfile.c
> >>> +++ b/mm/swapfile.c
> >>> @@ -1356,6 +1356,57 @@ void swap_free(swp_entry_t entry)
> >>> __swap_entry_free(p, entry);
> >>> }
> >>>
> >>> +/*
> >>> + * Free up the maximum number of swap entries at once to limit the
> >>> + * maximum kernel stack usage.
> >>> + */
> >>> +#define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : SWAPFILE_CLUSTER)
> >>> +
> >>> +/*
> >>> + * Called after swapping in a large folio, batched free swap entries
> >>> + * for this large folio, entry should be for the first subpage and
> >>> + * its offset is aligned with nr_pages
> >>> + */
> >>> +void swap_free_nr(swp_entry_t entry, int nr_pages)
> >>> +{
> >>> + int i, j;
> >>> + struct swap_cluster_info *ci;
> >>> + struct swap_info_struct *p;
> >>> + unsigned int type = swp_type(entry);
> >>> + unsigned long offset = swp_offset(entry);
> >>> + int batch_nr, remain_nr;
> >>> + DECLARE_BITMAP(usage, SWAP_BATCH_NR) = { 0 };
> >>> +
> >>> + /* all swap entries are within a cluster for mTHP */
> >>> + VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
> >>> +
> >>> + if (nr_pages == 1) {
> >>> + swap_free(entry);
> >>> + return;
> >>> + }
> >>> +
> >>> + remain_nr = nr_pages;
> >>> + p = _swap_info_get(entry);
> >>> + if (p) {
> >>
> >> nit: perhaps return early if (!p) ? Then you dedent the for() block.
> >
> > Agreed!
> >
> >>
> >>> + for (i = 0; i < nr_pages; i += batch_nr) {
> >>> + batch_nr = min_t(int, SWAP_BATCH_NR, remain_nr);
> >>> +
> >>> + ci = lock_cluster_or_swap_info(p, offset);
> >>> + for (j = 0; j < batch_nr; j++) {
> >>> + if (__swap_entry_free_locked(p, offset + i * SWAP_BATCH_NR + j, 1))
> >>> + __bitmap_set(usage, j, 1);
> >>> + }
> >>> + unlock_cluster_or_swap_info(p, ci);
> >>> +
> >>> + for_each_clear_bit(j, usage, batch_nr)
> >>> + free_swap_slot(swp_entry(type, offset + i * SWAP_BATCH_NR + j));
> >>> +
> >>
> >> nit: perhaps change to for (;;), and do the checks here to avoid clearing the
> >> bitmap on the last run:
> >>
> >> i += batch_nr;
> >> if (i < nr_pages)
> >> break;
Should be：
if (i >= nr_pages)
break;
> > Great, thank you for your advice!
>
> Or maybe leave the for() as is, but don't explicitly init the bitmap at the
> start of the function and instead call:
>
> bitmap_clear(usage, 0, SWAP_BATCH_NR);
>
> At the start of each loop?
Yeah, that's OK, actually these two ways are similar, both are to
reduce the number of bitmap_clear calls.
>
> >>
> >>> + bitmap_clear(usage, 0, SWAP_BATCH_NR);
> >>> + remain_nr -= batch_nr;
> >>> + }
> >>> + }
> >>> +}
> >>> +
> >>> /*
> >>> * Called after dropping swapcache to decrease refcnt to swap entries.
> >>> */
> >>
> >>
> >
> >
>
--
Thanks,
Chuanhua
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