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Message-ID: <CAMgjq7Akg4HOOPNamgMw+3cNc9-oFzT8Jv9r25Sk0EdGcR3SrA@mail.gmail.com>
Date: Tue, 10 Jun 2025 20:07:11 +0800
From: Kairui Song <ryncsn@...il.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Kemeng Shi <shikemeng@...weicloud.com>, Chris Li <chrisl@...nel.org>,
Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, Usama Arif <usamaarif642@...il.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/shmem, swap: fix softlockup with mTHP swapin
On Tue, Jun 10, 2025 at 2:16 AM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Mon, Jun 9, 2025 at 10:18 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > Following softlockup can be easily reproduced on my test machine with:
> >
> > echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
> > swapon /dev/zram0 # zram0 is a 48G swap device
> > mkdir -p /sys/fs/cgroup/memory/test
> > echo 1G > /sys/fs/cgroup/test/memory.max
> > echo $BASHPID > /sys/fs/cgroup/test/cgroup.procs
> > while true; do
> > dd if=/dev/zero of=/tmp/test.img bs=1M count=5120
> > cat /tmp/test.img > /dev/null
> > rm /tmp/test.img
> > done
> >
> > Then after a while:
> > watchdog: BUG: soft lockup - CPU#0 stuck for 763s! [cat:5787]
> > Modules linked in: zram virtiofs
> > CPU: 0 UID: 0 PID: 5787 Comm: cat Kdump: loaded Tainted: G L 6.15.0.orig-gf3021d9246bc-dirty #118 PREEMPT(voluntary)·
> > Tainted: [L]=SOFTLOCKUP
> > Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
> > RIP: 0010:mpol_shared_policy_lookup+0xd/0x70
> > Code: e9 b8 b4 ff ff 31 c0 c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 41 54 55 53 <48> 8b 1f 48 85 db 74 41 4c 8d 67 08 48 89 fb 48 89 f5 4c 89 e7 e8
> > RSP: 0018:ffffc90002b1fc28 EFLAGS: 00000202
> > RAX: 00000000001c20ca RBX: 0000000000724e1e RCX: 0000000000000001
> > RDX: ffff888118e214c8 RSI: 0000000000057d42 RDI: ffff888118e21518
> > RBP: 000000000002bec8 R08: 0000000000000001 R09: 0000000000000000
> > R10: 0000000000000bf4 R11: 0000000000000000 R12: 0000000000000001
> > R13: 00000000001c20ca R14: 00000000001c20ca R15: 0000000000000000
> > FS: 00007f03f995c740(0000) GS:ffff88a07ad9a000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f03f98f1000 CR3: 0000000144626004 CR4: 0000000000770eb0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > PKRU: 55555554
> > Call Trace:
> > <TASK>
> > shmem_alloc_folio+0x31/0xc0
> > shmem_swapin_folio+0x309/0xcf0
> > ? filemap_get_entry+0x117/0x1e0
> > ? xas_load+0xd/0xb0
> > ? filemap_get_entry+0x101/0x1e0
> > shmem_get_folio_gfp+0x2ed/0x5b0
> > shmem_file_read_iter+0x7f/0x2e0
> > vfs_read+0x252/0x330
> > ksys_read+0x68/0xf0
> > do_syscall_64+0x4c/0x1c0
> > entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > RIP: 0033:0x7f03f9a46991
> > Code: 00 48 8b 15 81 14 10 00 f7 d8 64 89 02 b8 ff ff ff ff eb bd e8 20 ad 01 00 f3 0f 1e fa 80 3d 35 97 10 00 00 74 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 4f c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec
> > RSP: 002b:00007fff3c52bd28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > RAX: ffffffffffffffda RBX: 0000000000040000 RCX: 00007f03f9a46991
> > RDX: 0000000000040000 RSI: 00007f03f98ba000 RDI: 0000000000000003
> > RBP: 00007fff3c52bd50 R08: 0000000000000000 R09: 00007f03f9b9a380
> > R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000040000
> > R13: 00007f03f98ba000 R14: 0000000000000003 R15: 0000000000000000
> > </TASK>
> >
> > The reason is simple, readahead brought some order 0 folio in swap
> > cache, and the swapin mTHP folio being allocated is in confict with it,
> > so swapcache_prepare fails and causes shmem_swap_alloc_folio to return
> > -EEXIST, and shmem simply retries again and again causing this loop.
> >
> > Fix it by applying a similar fix for anon mTHP swapin.
> >
> > The performance change is very slight, time of swapin 10g zero folios
> > with shmem (test for 12 times):
> > Before: 2.47s
> > After: 2.48s
> >
> > Fixes: 1dd44c0af4fa1 ("mm: shmem: skip swapcache for swapin of synchronous swap device")
> > Signed-off-by: Kairui Song <kasong@...cent.com>
>
> LGTM.
> Acked-by: Nhat Pham <nphamcs@...il.com>
>
> >
> > ---
> >
> > V1: https://lore.kernel.org/linux-mm/20250608192713.95875-1-ryncsn@gmail.com/
> > Updates:
> > - Move non_swapcache_batch check before swapcache_prepare, I was
> > expecting this could improve the performance, turns out it barely
> > helps and may even cause more overhead in some cases. [ Barry Song ]
> > - Remove zero map check, no need to do that for shmem [ Barry Song,
> > Baolin Wang ]
> > - Fix build bot error.
> >
> > mm/memory.c | 20 --------------------
> > mm/shmem.c | 4 +++-
> > mm/swap.h | 23 +++++++++++++++++++++++
> > 3 files changed, 26 insertions(+), 21 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 9ead7ab07e8e..3845ed068d74 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4313,26 +4313,6 @@ static struct folio *__alloc_swap_folio(struct vm_fault *vmf)
> > }
> >
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > -static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
> > -{
> > - struct swap_info_struct *si = swp_swap_info(entry);
> > - pgoff_t offset = swp_offset(entry);
> > - int i;
> > -
> > - /*
> > - * While allocating a large folio and doing swap_read_folio, which is
> > - * the case the being faulted pte doesn't have swapcache. We need to
> > - * ensure all PTEs have no cache as well, otherwise, we might go to
> > - * swap devices while the content is in swapcache.
> > - */
> > - for (i = 0; i < max_nr; i++) {
> > - if ((si->swap_map[offset + i] & SWAP_HAS_CACHE))
> > - return i;
> > - }
> > -
> > - return i;
> > -}
> > -
> > /*
> > * Check if the PTEs within a range are contiguous swap entries
> > * and have consistent swapcache, zeromap.
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 73182e904f9c..a4fdfbd086f1 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2256,6 +2256,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > folio = swap_cache_get_folio(swap, NULL, 0);
> > order = xa_get_order(&mapping->i_pages, index);
> > if (!folio) {
> > + int nr_pages = 1 << order;
> > bool fallback_order0 = false;
> >
> > /* Or update major stats only when swapin succeeds?? */
> > @@ -2271,7 +2272,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > * to swapin order-0 folio, as well as for zswap case.
>
> nit: maybe update the above comment?
Good idea. There are already some comments in non_swapcache_batch and
I've updated it for this commit, but mentioning it here definitely
helps more.
>
> > */
> > if (order > 0 && ((vma && unlikely(userfaultfd_armed(vma))) ||
> > - !zswap_never_enabled()))
> > + !zswap_never_enabled() ||
> > + non_swapcache_batch(swap, nr_pages) != nr_pages))
> > fallback_order0 = true;
> >
> > /* Skip swapcache for synchronous device. */
> > diff --git a/mm/swap.h b/mm/swap.h
> > index e87a0f19a0ee..911ad5ff0f89 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -108,6 +108,25 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr,
> > return find_next_bit(sis->zeromap, end, start) - start;
> > }
> >
>
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