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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] mm/pagewalk: fix race between concurrent split and refault Message-ID: <6c413f17-01c8-43fa-b5d5-45fe32dd14a8@lucifer.local> References: <20260325-pagewalk-check-pmd-refault-v2-1-707bff33bc60@akamai.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260325-pagewalk-check-pmd-refault-v2-1-707bff33bc60@akamai.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 83F5A1C0010 X-Stat-Signature: m3px1abgx6byy4by78mgaja7nybqqryh X-Rspam-User: X-HE-Tag: 1774451695-732667 X-HE-Meta: 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 TsiA7MZy FmZ110NLwW865DflP9IOa7wt3UikkBQhp2WXvwAMgzNyjrFcFMSwMu11nDTAnoeLaJeQMDZqaEwgmQzzX1BXe1nyyZbsX/ltZi8ko/q/ChQonQ1IGIBizRw4yVm5naduWKpYqeKpD2W1gVKBZM2qgAND284rzVhFF5BAZKRBrqkmEitrYQPPwNyk5CUMGhs4LrD66tDJrzS7QfLQQaY7kmNF3IrBgM2cbIiZzCwUo4XyYDpmEioopD7+GAAb/CMuDnS2H Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 25, 2026 at 10:59:16AM +0100, Max Boone via B4 Relay wrote: > From: Max Boone > > The splitting of a PUD entry in walk_pud_range() can race with > a concurrent thread refaulting the PUD leaf entry causing it to > try walking a PMD range that has disappeared. > > An example and reproduction of this is to try reading numa_maps of > a process while VFIO-PCI is setting up DMA (specifically the > vfio_pin_pages_remote call) on a large BAR for that process. > > This will trigger a kernel BUG: > vfio-pci 0000:03:00.0: enabling device (0000 -> 0002) > BUG: unable to handle page fault for address: ffffa23980000000 > PGD 0 P4D 0 > Oops: Oops: 0000 [#1] SMP NOPTI > ... > RIP: 0010:walk_pgd_range+0x3b5/0x7a0 > Code: 8d 43 ff 48 89 44 24 28 4d 89 ce 4d 8d a7 00 00 20 00 48 8b 4c 24 > 28 49 81 e4 00 00 e0 ff 49 8d 44 24 ff 48 39 c8 4c 0f 43 e3 <49> f7 06 > 9f ff ff ff 75 3b 48 8b 44 24 20 48 8b 40 28 48 85 c0 74 > RSP: 0018:ffffac23e1ecf808 EFLAGS: 00010287 > RAX: 00007f44c01fffff RBX: 00007f4500000000 RCX: 00007f44ffffffff > RDX: 0000000000000000 RSI: 000ffffffffff000 RDI: ffffffff93378fe0 > RBP: ffffac23e1ecf918 R08: 0000000000000004 R09: ffffa23980000000 > R10: 0000000000000020 R11: 0000000000000004 R12: 00007f44c0200000 > R13: 00007f44c0000000 R14: ffffa23980000000 R15: 00007f44c0000000 > FS: 00007fe884739580(0000) GS:ffff9b7d7a9c0000(0000) > knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: ffffa23980000000 CR3: 000000c0650e2005 CR4: 0000000000770ef0 > PKRU: 55555554 > Call Trace: > > __walk_page_range+0x195/0x1b0 > walk_page_vma+0x62/0xc0 > show_numa_map+0x12b/0x3b0 > seq_read_iter+0x297/0x440 > seq_read+0x11d/0x140 > vfs_read+0xc2/0x340 > ksys_read+0x5f/0xe0 > do_syscall_64+0x68/0x130 > ? get_page_from_freelist+0x5c2/0x17e0 > ? mas_store_prealloc+0x17e/0x360 > ? vma_set_page_prot+0x4c/0xa0 > ? __alloc_pages_noprof+0x14e/0x2d0 > ? __mod_memcg_lruvec_state+0x8d/0x140 > ? __lruvec_stat_mod_folio+0x76/0xb0 > ? __folio_mod_stat+0x26/0x80 > ? do_anonymous_page+0x705/0x900 > ? __handle_mm_fault+0xa8d/0x1000 > ? __count_memcg_events+0x53/0xf0 > ? handle_mm_fault+0xa5/0x360 > ? do_user_addr_fault+0x342/0x640 > ? arch_exit_to_user_mode_prepare.constprop.0+0x16/0xa0 > ? irqentry_exit_to_user_mode+0x24/0x100 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > RIP: 0033:0x7fe88464f47e > Code: c0 e9 b6 fe ff ff 50 48 8d 3d be 07 0b 00 e8 69 01 02 00 66 0f 1f > 84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 > f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28 > RSP: 002b:00007ffe6cd9a9b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 > RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fe88464f47e > RDX: 0000000000020000 RSI: 00007fe884543000 RDI: 0000000000000003 > RBP: 00007fe884543000 R08: 00007fe884542010 R09: 0000000000000000 > R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000 > R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000 > > > Fix this by validating the PUD entry in walk_pmd_range() using a stable > snapshot (pudp_get()). If the PUD is not present or is a leaf, retry the > walk via ACTION_AGAIN instead of descending further. This mirrors the > retry logic in walk_pte_range(), which lets walk_pmd_range() retry if > the PTE is not being got by pte_offset_map_lock(). > > Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support") > Cc: stable@vger.kernel.org > Co-developed-by: David Hildenbrand (Arm) > Signed-off-by: David Hildenbrand (Arm) > Signed-off-by: Max Boone LGTM, so: Reviewed-by: Lorenzo Stoakes (Oracle) (I reviewed before but maybe you didn't see that I was fine with you applying tag after nits addressed :) > --- > Changes in v2: > - extended the comment in walk_pmd_range with split/refault example. > - changed fixes, race not introduced by hugepage splitting but rather > with huge pfnmaps of BARs. > - clarified that the retry logic mirrors walk_pte_range instead of > walk_pmd_range. > - style changes (removed trailing newline) > - Link to v1: https://lore.kernel.org/r/20260317-pagewalk-check-pmd-refault-v1-1-f699a010f2b3@akamai.com Thanks, Lorenzo > --- > mm/pagewalk.c | 25 ++++++++++++++++++++++--- > 1 file changed, 22 insertions(+), 3 deletions(-) > > diff --git a/mm/pagewalk.c b/mm/pagewalk.c > index a94c401ab..4e7bcd975 100644 > --- a/mm/pagewalk.c > +++ b/mm/pagewalk.c > @@ -97,6 +97,7 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end, > static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, > struct mm_walk *walk) > { > + pud_t pudval = pudp_get(pud); > pmd_t *pmd; > unsigned long next; > const struct mm_walk_ops *ops = walk->ops; > @@ -105,6 +106,24 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, > int err = 0; > int depth = real_depth(3); > > + /* > + * For PTE handling, pte_offset_map_lock() takes care of checking > + * whether there actually is a page table. But it also has to be > + * very careful about concurrent page table reclaim. > + * > + * Similarly, we have to be careful here - a PUD entry that points > + * to a PMD table cannot go away, so we can just walk it. But if > + * it's something else, we need to ensure we didn't race something, > + * so need to retry. > + * > + * A pertinent example of this is a PUD refault after PUD split - > + * we will need to split again or risk accessing invalid memory. > + */ > + if (!pud_present(pudval) || pud_leaf(pudval)) { > + walk->action = ACTION_AGAIN; > + return 0; > + } > + > pmd = pmd_offset(pud, addr); > do { > again: > @@ -218,12 +237,12 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end, > else if (pud_leaf(*pud) || !pud_present(*pud)) > continue; /* Nothing to do. */ > > - if (pud_none(*pud)) > - goto again; > - > err = walk_pmd_range(pud, addr, next, walk); > if (err) > break; > + > + if (walk->action == ACTION_AGAIN) > + goto again; > } while (pud++, addr = next, addr != end); > > return err; > > --- > base-commit: b4f0dd314b39ea154f62f3bd3115ed0470f9f71e > change-id: 20260317-pagewalk-check-pmd-refault-de8f14fbe6a5 > > Best regards, > -- > Max Boone > >