The patch titled Subject: mm/huge_memory: do not clobber swp_entry_t during THP split has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-huge_memory-do-not-clobber-swp_entry_t-during-thp-split.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Mel Gorman mgorman@techsingularity.net Subject: mm/huge_memory: do not clobber swp_entry_t during THP split Date: Wed, 19 Oct 2022 14:41:56 +0100

The following has been observed when running stressng mmap since commit b653db77350c ("mm: Clear page->private when splitting or migrating a page")

watchdog: BUG: soft lockup - CPU#75 stuck for 26s! [stress-ng:9546] CPU: 75 PID: 9546 Comm: stress-ng Tainted: G E 6.0.0-revert-b653db77-fix+ #29 0357d79b60fb09775f678e4f3f64ef0579ad1374 Hardware name: SGI.COM C2112-4GP3/X10DRT-P-Series, BIOS 2.0a 05/09/2016 RIP: 0010:xas_descend+0x28/0x80 Code: cc cc 0f b6 0e 48 8b 57 08 48 d3 ea 83 e2 3f 89 d0 48 83 c0 04 48 8b 44 c6 08 48 89 77 18 48 89 c1 83 e1 03 48 83 f9 02 75 08 <48> 3d fd 00 00 00 76 08 88 57 12 c3 cc cc cc cc 48 c1 e8 02 89 c2 RSP: 0018:ffffbbf02a2236a8 EFLAGS: 00000246 RAX: ffff9cab7d6a0002 RBX: ffffe04b0af88040 RCX: 0000000000000002 RDX: 0000000000000030 RSI: ffff9cab60509b60 RDI: ffffbbf02a2236c0 RBP: 0000000000000000 R08: ffff9cab60509b60 R09: ffffbbf02a2236c0 R10: 0000000000000001 R11: ffffbbf02a223698 R12: 0000000000000000 R13: ffff9cab4e28da80 R14: 0000000000039c01 R15: ffff9cab4e28da88 FS: 00007fab89b85e40(0000) GS:ffff9cea3fcc0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fab84e00000 CR3: 00000040b73a4003 CR4: 00000000003706e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> xas_load+0x3a/0x50 __filemap_get_folio+0x80/0x370 ? put_swap_page+0x163/0x360 pagecache_get_page+0x13/0x90 __try_to_reclaim_swap+0x50/0x190 scan_swap_map_slots+0x31e/0x670 get_swap_pages+0x226/0x3c0 folio_alloc_swap+0x1cc/0x240 add_to_swap+0x14/0x70 shrink_page_list+0x968/0xbc0 reclaim_page_list+0x70/0xf0 reclaim_pages+0xdd/0x120 madvise_cold_or_pageout_pte_range+0x814/0xf30 walk_pgd_range+0x637/0xa30 __walk_page_range+0x142/0x170 walk_page_range+0x146/0x170 madvise_pageout+0xb7/0x280 ? asm_common_interrupt+0x22/0x40 madvise_vma_behavior+0x3b7/0xac0 ? find_vma+0x4a/0x70 ? find_vma+0x64/0x70 ? madvise_vma_anon_name+0x40/0x40 madvise_walk_vmas+0xa6/0x130 do_madvise+0x2f4/0x360 __x64_sys_madvise+0x26/0x30 do_syscall_64+0x5b/0x80 ? do_syscall_64+0x67/0x80 ? syscall_exit_to_user_mode+0x17/0x40 ? do_syscall_64+0x67/0x80 ? syscall_exit_to_user_mode+0x17/0x40 ? do_syscall_64+0x67/0x80 ? do_syscall_64+0x67/0x80 ? common_interrupt+0x8b/0xa0 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The problem can be reproduced with the mmtests config config-workload-stressng-mmap. It does not always happen and when it triggers is variable but it has happened on multiple machines.

The intent of commit b653db77350c patch was to avoid the case where PG_private is clear but folio->private is not-NULL. However, THP tail pages uses page->private for "swp_entry_t if folio_test_swapcache()" as stated in the documentation for struct folio. This patch only clobbers page->private for tail pages if the head page was not in swapcache and warns once if page->private had an unexpected value.

Link: https://lkml.kernel.org/r/20221019134156.zjyyn5aownakvztf@techsingularity.ne... Fixes: b653db77350c ("mm: Clear page->private when splitting or migrating a page") Signed-off-by: Mel Gorman mgorman@techsingularity.net Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Mel Gorman mgorman@techsingularity.net Cc: Yang Shi shy828301@gmail.com Cc: Brian Foster bfoster@redhat.com Cc: Dan Streetman ddstreet@ieee.org Cc: Miaohe Lin linmiaohe@huawei.com Cc: Oleksandr Natalenko oleksandr@natalenko.name Cc: Seth Jennings sjenning@redhat.com Cc: Vitaly Wool vitaly.wool@konsulko.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/huge_memory.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/mm/huge_memory.c~mm-huge_memory-do-not-clobber-swp_entry_t-during-thp-split +++ a/mm/huge_memory.c @@ -2455,7 +2455,16 @@ static void __split_huge_page_tail(struc page_tail); page_tail->mapping = head->mapping; page_tail->index = head->index + tail; - page_tail->private = 0; + + /* + * page->private should not be set in tail pages with the exception + * of swap cache pages that store the swp_entry_t in tail pages. + * Fix up and warn once if private is unexpectedly set. + */ + if (!folio_test_swapcache(page_folio(head))) { + VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head); + page_tail->private = 0; + }

/* Page flags must be visible before we make the page non-compound. */ smp_wmb(); _

Patches currently in -mm which might be from mgorman@techsingularity.net are

mm-huge_memory-do-not-clobber-swp_entry_t-during-thp-split.patch