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Message-ID: <YOvpVRSMJe8NQuS2@dhcp22.suse.cz>
Date: Mon, 12 Jul 2021 09:03:49 +0200
From: Michal Hocko <mhocko@...e.com>
To: Evan Green <evgreen@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Pavel Machek <pavel@....cz>, Alex Shi <alexs@...nel.org>,
Alistair Popple <apopple@...dia.com>,
Jens Axboe <axboe@...nel.dk>,
Johannes Weiner <hannes@...xchg.org>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Miaohe Lin <linmiaohe@...wei.com>,
Minchan Kim <minchan@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v2] mm: Enable suspend-only swap spaces
[Cc linux-api]
On Fri 09-07-21 10:50:48, Evan Green wrote:
> Currently it's not possible to enable hibernation without also enabling
> generic swap for a given swap area. These two use cases are not the
> same. For example there may be users who want to enable hibernation,
> but whose drives don't have the write endurance for generic swap
> activities.
>
> Add a new SWAP_FLAG_NOSWAP that adds a swap region but refuses to allow
> generic swapping to it. This region can still be wired up for use in
> suspend-to-disk activities, but will never have regular pages swapped to
> it.
Could you expand some more on why a strict exclusion is really
necessary? I do understand that one might not want to have swap storage
available all the time but considering that swapon is really a light
operation so something like the following should be a reasonable
workaround, no?
swapon storage/file
s2disk
swapoff storage
> Swap regions with SWAP_FLAG_NOSWAP set will not appear in /proc/meminfo
> under SwapTotal and SwapFree, since they are not usable as general swap.
>
> Signed-off-by: Evan Green <evgreen@...omium.org>
> ---
>
> Changes in v2:
> - NOSWAP regions should not contribute to Swap stats in /proc/meminfo.
> [David]
> - Adjusted comment of SWAP_FLAG_NOSWAP [Pavel]
> - Note: Opted not to take Pavel's tag since enough has changed in this
> revision to warrant another look.
> - Call swap_entry_free() in swap_free to avoid NOSWAP leaks back into
> the general pool via swap_slots_cache [me].
>
> include/linux/swap.h | 4 +++-
> mm/swapfile.c | 52 +++++++++++++++++++++++++++++++-------------
> 2 files changed, 40 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 6f5a43251593c8..995466c9f16a76 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -28,10 +28,11 @@ struct pagevec;
> #define SWAP_FLAG_DISCARD 0x10000 /* enable discard for swap */
> #define SWAP_FLAG_DISCARD_ONCE 0x20000 /* discard swap area at swapon-time */
> #define SWAP_FLAG_DISCARD_PAGES 0x40000 /* discard page-clusters after use */
> +#define SWAP_FLAG_NOSWAP 0x80000 /* use only for hibernate, not swap */
>
> #define SWAP_FLAGS_VALID (SWAP_FLAG_PRIO_MASK | SWAP_FLAG_PREFER | \
> SWAP_FLAG_DISCARD | SWAP_FLAG_DISCARD_ONCE | \
> - SWAP_FLAG_DISCARD_PAGES)
> + SWAP_FLAG_DISCARD_PAGES | SWAP_FLAG_NOSWAP)
> #define SWAP_BATCH 64
>
> static inline int current_is_kswapd(void)
> @@ -182,6 +183,7 @@ enum {
> SWP_PAGE_DISCARD = (1 << 10), /* freed swap page-cluster discards */
> SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */
> SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */
> + SWP_NOSWAP = (1 << 13), /* use only for suspend, not swap */
> /* add others here before... */
> SWP_SCANNING = (1 << 14), /* refcount in scan_swap_map */
> };
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 1e07d1c776f2ae..d9ab39d32e55ec 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -697,7 +697,8 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
> if (si->inuse_pages == si->pages) {
> si->lowest_bit = si->max;
> si->highest_bit = 0;
> - del_from_avail_list(si);
> + if (!(si->flags & SWP_NOSWAP))
> + del_from_avail_list(si);
> }
> }
>
> @@ -726,10 +727,12 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> bool was_full = !si->highest_bit;
>
> WRITE_ONCE(si->highest_bit, end);
> - if (was_full && (si->flags & SWP_WRITEOK))
> + if (was_full &&
> + ((si->flags & (SWP_WRITEOK | SWP_NOSWAP)) == SWP_WRITEOK))
> add_to_avail_list(si);
> }
> - atomic_long_add(nr_entries, &nr_swap_pages);
> + if (!(si->flags & SWP_NOSWAP))
> + atomic_long_add(nr_entries, &nr_swap_pages);
> si->inuse_pages -= nr_entries;
> if (si->flags & SWP_BLKDEV)
> swap_slot_free_notify =
> @@ -1078,6 +1081,9 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> WARN(!(si->flags & SWP_WRITEOK),
> "swap_info %d in list but !SWP_WRITEOK\n",
> si->type);
> + WARN((si->flags & SWP_NOSWAP),
> + "swap_info %d in list but SWP_NOSWAP\n",
> + si->type);
> __del_from_avail_list(si);
> spin_unlock(&si->lock);
> goto nextsi;
> @@ -1338,8 +1344,12 @@ void swap_free(swp_entry_t entry)
> struct swap_info_struct *p;
>
> p = _swap_info_get(entry);
> - if (p)
> - __swap_entry_free(p, entry);
> + if (p) {
> + if (p->flags & SWP_NOSWAP)
> + swap_entry_free(p, entry);
> + else
> + __swap_entry_free(p, entry);
> + }
> }
>
> /*
> @@ -1783,8 +1793,10 @@ swp_entry_t get_swap_page_of_type(int type)
>
> /* This is called for allocating swap entry, not cache */
> spin_lock(&si->lock);
> - if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry))
> - atomic_long_dec(&nr_swap_pages);
> + if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry)) {
> + if (!(si->flags & SWP_NOSWAP))
> + atomic_long_dec(&nr_swap_pages);
> + }
> spin_unlock(&si->lock);
> fail:
> return entry;
> @@ -2454,8 +2466,6 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
> static void _enable_swap_info(struct swap_info_struct *p)
> {
> p->flags |= SWP_WRITEOK;
> - atomic_long_add(p->pages, &nr_swap_pages);
> - total_swap_pages += p->pages;
>
> assert_spin_locked(&swap_lock);
> /*
> @@ -2469,7 +2479,11 @@ static void _enable_swap_info(struct swap_info_struct *p)
> * swap_info_struct.
> */
> plist_add(&p->list, &swap_active_head);
> - add_to_avail_list(p);
> + if (!(p->flags & SWP_NOSWAP)) {
> + atomic_long_add(p->pages, &nr_swap_pages);
> + total_swap_pages += p->pages;
> + add_to_avail_list(p);
> + }
> }
>
> static void enable_swap_info(struct swap_info_struct *p, int prio,
> @@ -2564,7 +2578,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> spin_unlock(&swap_lock);
> goto out_dput;
> }
> - del_from_avail_list(p);
> + if (!(p->flags & SWP_NOSWAP))
> + del_from_avail_list(p);
> +
> spin_lock(&p->lock);
> if (p->prio < 0) {
> struct swap_info_struct *si = p;
> @@ -2581,8 +2597,10 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> least_priority++;
> }
> plist_del(&p->list, &swap_active_head);
> - atomic_long_sub(p->pages, &nr_swap_pages);
> - total_swap_pages -= p->pages;
> + if (!(p->flags & SWP_NOSWAP)) {
> + atomic_long_sub(p->pages, &nr_swap_pages);
> + total_swap_pages -= p->pages;
> + }
> p->flags &= ~SWP_WRITEOK;
> spin_unlock(&p->lock);
> spin_unlock(&swap_lock);
> @@ -3329,16 +3347,20 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> if (swap_flags & SWAP_FLAG_PREFER)
> prio =
> (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
> +
> + if (swap_flags & SWAP_FLAG_NOSWAP)
> + p->flags |= SWP_NOSWAP;
> enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
>
> - pr_info("Adding %uk swap on %s. Priority:%d extents:%d across:%lluk %s%s%s%s%s\n",
> + pr_info("Adding %uk swap on %s. Priority:%d extents:%d across:%lluk %s%s%s%s%s%s\n",
> p->pages<<(PAGE_SHIFT-10), name->name, p->prio,
> nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
> (p->flags & SWP_SOLIDSTATE) ? "SS" : "",
> (p->flags & SWP_DISCARDABLE) ? "D" : "",
> (p->flags & SWP_AREA_DISCARD) ? "s" : "",
> (p->flags & SWP_PAGE_DISCARD) ? "c" : "",
> - (frontswap_map) ? "FS" : "");
> + (frontswap_map) ? "FS" : "",
> + (p->flags & SWP_NOSWAP) ? "N" : "");
>
> mutex_unlock(&swapon_mutex);
> atomic_inc(&proc_poll_event);
> --
> 2.31.0
--
Michal Hocko
SUSE Labs
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