On 8/15/21 12:14 PM, Vlastimil Babka wrote: > On 8/5/21 5:19 PM, Vlastimil Babka wrote: >> As another step of shortening irq disabled sections in ___slab_alloc(), delay >> disabling irqs until we pass the initial checks if there is a cached percpu >> slab and it's suitable for our allocation. >> >> Now we have to recheck c->page after actually disabling irqs as an allocation >> in irq handler might have replaced it. > > Please add an extra paragraph that related to the fixup below (which I > assume will be squashed as usual): > > Because we call pfmemalloc_match() as one of the checks, we might hit > VM_BUG_ON_PAGE(!PageSlab(page)) in PageSlabPfmemalloc in case we get > interrupted and the page is freed. Thus introduce a > pfmemalloc_match_unsafe() variant that lacks the PageSlab check. > >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> >> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > > And the fixup: Oops, renaming snafu. Again. ----8<---- >From bf81bca38b127a8d717978467cf7264580c81248 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka <vbabka@xxxxxxx> Date: Sun, 15 Aug 2021 11:49:46 +0200 Subject: [PATCH] mm, slub: prevent VM_BUG_ON in PageSlabPfmemalloc from ___slab_alloc Clark Williams reported [1] a VM_BUG_ON in PageSlabPfmemalloc: page:000000009ac5dd73 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1ab3db flags: 0x17ffffc0000000(node=0|zone=2|lastcpupid=0x1fffff) raw: 0017ffffc0000000 ffffee1286aceb88 ffffee1287b66288 0000000000000000 raw: 0000000000000000 0000000000100000 00000000ffffffff 0000000000000000 page dumped because: VM_BUG_ON_PAGE(!PageSlab(page)) ------------[ cut here ]------------ kernel BUG at include/linux/page-flags.h:814! invalid opcode: 0000 [#1] PREEMPT_RT SMP PTI CPU: 3 PID: 12345 Comm: hackbench Not tainted 5.14.0-rc5-rt8+ #12 Hardware name: /NUC5i7RYB, BIOS RYBDWi35.86A.0359.2016.0906.1028 09/06/2016 RIP: 0010:___slab_alloc+0x340/0x940 Code: c6 48 0f a3 05 b1 7b 57 03 72 99 c7 85 78 ff ff ff ff ff ff ff 48 8b 7d 88 e9 8d fd ff ff 48 c7 c6 50 5a 7c b0 e> RSP: 0018:ffffba1c4a8b7ab0 EFLAGS: 00010293 RAX: 0000000000000000 RBX: 0000000000000002 RCX: ffff9bb765118000 RDX: 0000000000000000 RSI: ffffffffaf426050 RDI: 00000000ffffffff RBP: ffffba1c4a8b7b70 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: ffff9bb7410d3600 R13: 0000000000400cc0 R14: 00000000001f7770 R15: ffff9bbe76df7770 FS: 00007f474b1be740(0000) GS:ffff9bbe76c00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f60c04bdaf8 CR3: 0000000124f3a003 CR4: 00000000003706e0 Call Trace: ? __alloc_skb+0x1db/0x270 ? __alloc_skb+0x1db/0x270 ? kmem_cache_alloc_node+0xa4/0x2b0 kmem_cache_alloc_node+0xa4/0x2b0 __alloc_skb+0x1db/0x270 alloc_skb_with_frags+0x64/0x250 sock_alloc_send_pskb+0x260/0x2b0 ? bpf_lsm_socket_getpeersec_dgram+0xa/0x10 unix_stream_sendmsg+0x27c/0x550 ? unix_seqpacket_recvmsg+0x60/0x60 sock_sendmsg+0xbd/0xd0 sock_write_iter+0xb9/0x120 new_sync_write+0x175/0x200 vfs_write+0x3c4/0x510 ksys_write+0xc9/0x110 do_syscall_64+0x3b/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae The problem is that we are opportunistically checking flags on a page in irq enabled section. If we are interrupted and the page is freed, it's not an issue as we detect it after disabling irqs. But on kernels with CONFIG_DEBUG_VM. The check for PageSlab flag in PageSlabPfmemalloc() can fail. Fix this by creating an "unsafe" version of the check that doesn't check PageSlab. This is a fixup for mmotm patch mm-slub-do-initial-checks-in-___slab_alloc-with-irqs-enabled.patch [1] https://lore.kernel.org/lkml/20210812151803.52f84aaf@xxxxxxxxxxx/ Reported-by: Clark Williams <williams@xxxxxxxxxx> Tested-by: Mike Galbraith <efault@xxxxxx> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> --- include/linux/page-flags.h | 9 +++++++++ mm/slub.c | 15 ++++++++++++++- 2 files changed, 23 insertions(+), 1 deletion(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 5922031ffab6..7fda4fb85bdc 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -815,6 +815,15 @@ static inline int PageSlabPfmemalloc(struct page *page) return PageActive(page); } +/* + * A version of PageSlabPfmemalloc() for opportunistic checks where the page + * might have been freed under us and not be a PageSlab anymore. + */ +static inline int __PageSlabPfmemalloc(struct page *page) +{ + return PageActive(page); +} + static inline void SetPageSlabPfmemalloc(struct page *page) { VM_BUG_ON_PAGE(!PageSlab(page), page); diff --git a/mm/slub.c b/mm/slub.c index 7eb06fe9d7a0..d60d48c35f98 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2603,6 +2603,19 @@ static inline bool pfmemalloc_match(struct page *page, gfp_t gfpflags) return true; } +/* + * A variant of pfmemalloc_match() that tests page flags without asserting + * PageSlab. Intended for opportunistic checks before taking a lock and + * rechecking that nobody else freed the page under us. + */ +static inline bool pfmemalloc_match_unsafe(struct page *page, gfp_t gfpflags) +{ + if (unlikely(__PageSlabPfmemalloc(page))) + return gfp_pfmemalloc_allowed(gfpflags); + + return true; +} + /* * Check the page->freelist of a page and either transfer the freelist to the * per cpu freelist or deactivate the page. @@ -2704,7 +2717,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, * PFMEMALLOC but right now, we are losing the pfmemalloc * information when the page leaves the per-cpu allocator */ - if (unlikely(!pfmemalloc_match(page, gfpflags))) + if (unlikely(!pfmemalloc_match_unsafe(page, gfpflags))) goto deactivate_slab; /* must check again c->page in case IRQ handler changed it */ -- 2.32.0
