| Message ID | 20230630013203.1955064-1-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/hugetlb.c: fix a bug within a BUG(): inconsistent pte comparison | expand |
On Thu, Jun 29, 2023 at 9:32 PM John Hubbard <jhubbard@nvidia.com> wrote: > > The following crash happens for me when running the -mm selftests > (below). Specifically, it happens while running the uffd-stress > subtests: > > kernel BUG at mm/hugetlb.c:7249! > invalid opcode: 0000 [#1] PREEMPT SMP NOPTI > CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109 > Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018 > RIP: 0010:huge_pte_alloc+0x12c/0x1a0 > ... > Call Trace: > <TASK> > ? __die_body+0x63/0xb0 > ? die+0x9f/0xc0 > ? do_trap+0xab/0x180 > ? huge_pte_alloc+0x12c/0x1a0 > ? do_error_trap+0xc6/0x110 > ? huge_pte_alloc+0x12c/0x1a0 > ? handle_invalid_op+0x2c/0x40 > ? huge_pte_alloc+0x12c/0x1a0 > ? exc_invalid_op+0x33/0x50 > ? asm_exc_invalid_op+0x16/0x20 > ? __pfx_put_prev_task_idle+0x10/0x10 > ? huge_pte_alloc+0x12c/0x1a0 > hugetlb_fault+0x1a3/0x1120 > ? finish_task_switch+0xb3/0x2a0 > ? lock_is_held_type+0xdb/0x150 > handle_mm_fault+0xb8a/0xd40 > ? find_vma+0x5d/0xa0 > do_user_addr_fault+0x257/0x5d0 > exc_page_fault+0x7b/0x1f0 > asm_exc_page_fault+0x22/0x30 > > That happens because a BUG() statement in huge_pte_alloc() attempts to > check that a pte, if present, is a hugetlb pte, but it does so in a > non-lockless-safe manner that leads to a false BUG() report. > > We got here due to a couple of bugs, each of which by itself was not > quite enough to cause a problem: > > First of all, before commit c33c794828f2("mm: ptep_get() conversion"), > the BUG() statement in huge_pte_alloc() was itself fragile: it relied > upon compiler behavior to only read the pte once, despite using it twice > in the same conditional. > > Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that > delicate situation, by causing all direct pte reads to be done via > READ_ONCE(). And so READ_ONCE() got called twice within the same BUG() > conditional, leading to comparing (potentially, occasionally) different > versions of the pte, and thus to false BUG() reports. > > Fix this by taking a single snapshot of the pte before using it in the > BUG conditional. > > Now, that commit is only partially to blame here but, people doing > bisections will invariably land there, so this will help them find a fix > for a real crash. And also, the previous behavior was unlikely to ever > expose this bug--it was fragile, yet not actually broken. > > So that's why I chose this commit for the Fixes tag, rather than the > commit that created the original BUG() statement. > > Fixes: c33c794828f2 ("mm: ptep_get() conversion") Hi John, Good catch, and thanks for the detailed explanation. It looks like riscv and powerpc have equivalent problems in their huge_pte_alloc implementations, perhaps it's worth taking a look at those. (riscv looks like it has precisely the same problem except it's a WARN, but powerpc looks more interesting.) Either way, Acked-by: James Houghton <jthoughton@google.com>
On 6/29/23 18:50, James Houghton wrote: > Hi John, > > Good catch, and thanks for the detailed explanation. It looks like > riscv and powerpc have equivalent problems in their huge_pte_alloc > implementations, perhaps it's worth taking a look at those. (riscv > looks like it has precisely the same problem except it's a WARN, but > powerpc looks more interesting.) > > Either way, > > Acked-by: James Houghton <jthoughton@google.com> Thanks for the ack, and yes, riscv and powerpc do look like the same problem to me, too. I'm glad you thought to look there, as I have fallen into tunnel vision while investigating this, heh. OK, as long as I'm here, let me fix those up, too. I'll make those a separate patch, because they are still in the "fragile" state, rather than actually at the full crash state. :) thanks,
> On Jun 30, 2023, at 09:32, John Hubbard <jhubbard@nvidia.com> wrote: > > The following crash happens for me when running the -mm selftests > (below). Specifically, it happens while running the uffd-stress > subtests: > > kernel BUG at mm/hugetlb.c:7249! > invalid opcode: 0000 [#1] PREEMPT SMP NOPTI > CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109 > Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018 > RIP: 0010:huge_pte_alloc+0x12c/0x1a0 > ... > Call Trace: > <TASK> > ? __die_body+0x63/0xb0 > ? die+0x9f/0xc0 > ? do_trap+0xab/0x180 > ? huge_pte_alloc+0x12c/0x1a0 > ? do_error_trap+0xc6/0x110 > ? huge_pte_alloc+0x12c/0x1a0 > ? handle_invalid_op+0x2c/0x40 > ? huge_pte_alloc+0x12c/0x1a0 > ? exc_invalid_op+0x33/0x50 > ? asm_exc_invalid_op+0x16/0x20 > ? __pfx_put_prev_task_idle+0x10/0x10 > ? huge_pte_alloc+0x12c/0x1a0 > hugetlb_fault+0x1a3/0x1120 > ? finish_task_switch+0xb3/0x2a0 > ? lock_is_held_type+0xdb/0x150 > handle_mm_fault+0xb8a/0xd40 > ? find_vma+0x5d/0xa0 > do_user_addr_fault+0x257/0x5d0 > exc_page_fault+0x7b/0x1f0 > asm_exc_page_fault+0x22/0x30 > > That happens because a BUG() statement in huge_pte_alloc() attempts to > check that a pte, if present, is a hugetlb pte, but it does so in a > non-lockless-safe manner that leads to a false BUG() report. > > We got here due to a couple of bugs, each of which by itself was not > quite enough to cause a problem: > > First of all, before commit c33c794828f2("mm: ptep_get() conversion"), > the BUG() statement in huge_pte_alloc() was itself fragile: it relied > upon compiler behavior to only read the pte once, despite using it twice > in the same conditional. > > Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that > delicate situation, by causing all direct pte reads to be done via > READ_ONCE(). And so READ_ONCE() got called twice within the same BUG() > conditional, leading to comparing (potentially, occasionally) different > versions of the pte, and thus to false BUG() reports. > > Fix this by taking a single snapshot of the pte before using it in the > BUG conditional. > > Now, that commit is only partially to blame here but, people doing > bisections will invariably land there, so this will help them find a fix > for a real crash. And also, the previous behavior was unlikely to ever > expose this bug--it was fragile, yet not actually broken. > > So that's why I chose this commit for the Fixes tag, rather than the > commit that created the original BUG() statement. > > Fixes: c33c794828f2 ("mm: ptep_get() conversion") > Cc: Adrian Hunter <adrian.hunter@intel.com> > Cc: Al Viro <viro@zeniv.linux.org.uk> > Cc: Alex Williamson <alex.williamson@redhat.com> > Cc: Alexander Potapenko <glider@google.com> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Andrey Konovalov <andreyknvl@gmail.com> > Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> > Cc: Christian Brauner <brauner@kernel.org> > Cc: Christoph Hellwig <hch@infradead.org> > Cc: Daniel Vetter <daniel@ffwll.ch> > Cc: Dave Airlie <airlied@gmail.com> > Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com> > Cc: Dmitry Vyukov <dvyukov@google.com> > Cc: Ian Rogers <irogers@google.com> > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Jiri Olsa <jolsa@kernel.org> > Cc: Johannes Weiner <hannes@cmpxchg.org> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Cc: Lorenzo Stoakes <lstoakes@gmail.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Miaohe Lin <linmiaohe@huawei.com> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Mike Kravetz <mike.kravetz@oracle.com> > Cc: Mike Rapoport (IBM) <rppt@kernel.org> > Cc: Muchun Song <muchun.song@linux.dev> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Naoya Horiguchi <naoya.horiguchi@nec.com> > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> > Cc: Pavel Tatashin <pasha.tatashin@soleen.com> > Cc: Roman Gushchin <roman.gushchin@linux.dev> > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: SeongJae Park <sj@kernel.org> > Cc: Shakeel Butt <shakeelb@google.com> > Cc: Uladzislau Rezki (Sony) <urezki@gmail.com> > Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> > Cc: Yu Zhao <yuzhao@google.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> Acked-by: Muchun Song <songmuchun@bytedance.com> Thanks.
On 30/06/2023 02:32, John Hubbard wrote: > The following crash happens for me when running the -mm selftests > (below). Specifically, it happens while running the uffd-stress > subtests: > > kernel BUG at mm/hugetlb.c:7249! > invalid opcode: 0000 [#1] PREEMPT SMP NOPTI > CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109 > Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018 > RIP: 0010:huge_pte_alloc+0x12c/0x1a0 > ... > Call Trace: > <TASK> > ? __die_body+0x63/0xb0 > ? die+0x9f/0xc0 > ? do_trap+0xab/0x180 > ? huge_pte_alloc+0x12c/0x1a0 > ? do_error_trap+0xc6/0x110 > ? huge_pte_alloc+0x12c/0x1a0 > ? handle_invalid_op+0x2c/0x40 > ? huge_pte_alloc+0x12c/0x1a0 > ? exc_invalid_op+0x33/0x50 > ? asm_exc_invalid_op+0x16/0x20 > ? __pfx_put_prev_task_idle+0x10/0x10 > ? huge_pte_alloc+0x12c/0x1a0 > hugetlb_fault+0x1a3/0x1120 > ? finish_task_switch+0xb3/0x2a0 > ? lock_is_held_type+0xdb/0x150 > handle_mm_fault+0xb8a/0xd40 > ? find_vma+0x5d/0xa0 > do_user_addr_fault+0x257/0x5d0 > exc_page_fault+0x7b/0x1f0 > asm_exc_page_fault+0x22/0x30 > > That happens because a BUG() statement in huge_pte_alloc() attempts to > check that a pte, if present, is a hugetlb pte, but it does so in a > non-lockless-safe manner that leads to a false BUG() report. > > We got here due to a couple of bugs, each of which by itself was not > quite enough to cause a problem: > > First of all, before commit c33c794828f2("mm: ptep_get() conversion"), > the BUG() statement in huge_pte_alloc() was itself fragile: it relied > upon compiler behavior to only read the pte once, despite using it twice > in the same conditional. > > Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that > delicate situation, by causing all direct pte reads to be done via > READ_ONCE(). And so READ_ONCE() got called twice within the same BUG() > conditional, leading to comparing (potentially, occasionally) different > versions of the pte, and thus to false BUG() reports. Thanks for finding and fixing this - sorry for the issue. FWIW, I've re-reviewed the whole ptep_get conversion patch looking for other instances of this pattern - I didn't spot any other issues. > > Fix this by taking a single snapshot of the pte before using it in the > BUG conditional. > > Now, that commit is only partially to blame here but, people doing > bisections will invariably land there, so this will help them find a fix > for a real crash. And also, the previous behavior was unlikely to ever > expose this bug--it was fragile, yet not actually broken. > > So that's why I chose this commit for the Fixes tag, rather than the > commit that created the original BUG() statement. > > Fixes: c33c794828f2 ("mm: ptep_get() conversion") > Cc: Adrian Hunter <adrian.hunter@intel.com> > Cc: Al Viro <viro@zeniv.linux.org.uk> > Cc: Alex Williamson <alex.williamson@redhat.com> > Cc: Alexander Potapenko <glider@google.com> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Andrey Konovalov <andreyknvl@gmail.com> > Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> > Cc: Christian Brauner <brauner@kernel.org> > Cc: Christoph Hellwig <hch@infradead.org> > Cc: Daniel Vetter <daniel@ffwll.ch> > Cc: Dave Airlie <airlied@gmail.com> > Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com> > Cc: Dmitry Vyukov <dvyukov@google.com> > Cc: Ian Rogers <irogers@google.com> > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Jiri Olsa <jolsa@kernel.org> > Cc: Johannes Weiner <hannes@cmpxchg.org> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Cc: Lorenzo Stoakes <lstoakes@gmail.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Miaohe Lin <linmiaohe@huawei.com> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Mike Kravetz <mike.kravetz@oracle.com> > Cc: Mike Rapoport (IBM) <rppt@kernel.org> > Cc: Muchun Song <muchun.song@linux.dev> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Naoya Horiguchi <naoya.horiguchi@nec.com> > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> > Cc: Pavel Tatashin <pasha.tatashin@soleen.com> > Cc: Roman Gushchin <roman.gushchin@linux.dev> > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: SeongJae Park <sj@kernel.org> > Cc: Shakeel Butt <shakeelb@google.com> > Cc: Uladzislau Rezki (Sony) <urezki@gmail.com> > Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> > Cc: Yu Zhao <yuzhao@google.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> > --- > mm/hugetlb.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index bce28cca73a1..73fbeb8f979f 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, > pte = (pte_t *)pmd_alloc(mm, pud, addr); > } > } > - BUG_ON(pte && pte_present(ptep_get(pte)) && !pte_huge(ptep_get(pte))); > + > + if (pte) { > + pte_t pteval = ptep_get(pte); Given the PTL is not held here, I think this should technically be ptep_get_lockless()? Thanks, Ryan > + > + BUG_ON(pte_present(pteval) && !pte_huge(pteval)); > + } > > return pte; > } > > base-commit: bf1fa6f15553df04f2bdd06190ccd5f388ab0777
On 6/30/23 03:07, Ryan Roberts wrote: ... >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index bce28cca73a1..73fbeb8f979f 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, >> pte = (pte_t *)pmd_alloc(mm, pud, addr); >> } >> } >> - BUG_ON(pte && pte_present(ptep_get(pte)) && !pte_huge(ptep_get(pte))); >> + >> + if (pte) { >> + pte_t pteval = ptep_get(pte); > > Given the PTL is not held here, I think this should technically be > ptep_get_lockless()? > Oh yes. I'll change it to that and send a v2, thanks! thanks,
On 06/29/23 18:32, John Hubbard wrote: > The following crash happens for me when running the -mm selftests > (below). Specifically, it happens while running the uffd-stress > subtests: > > kernel BUG at mm/hugetlb.c:7249! > invalid opcode: 0000 [#1] PREEMPT SMP NOPTI > CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109 > Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018 > RIP: 0010:huge_pte_alloc+0x12c/0x1a0 > ... > Call Trace: > <TASK> > ? __die_body+0x63/0xb0 > ? die+0x9f/0xc0 > ? do_trap+0xab/0x180 > ? huge_pte_alloc+0x12c/0x1a0 > ? do_error_trap+0xc6/0x110 > ? huge_pte_alloc+0x12c/0x1a0 > ? handle_invalid_op+0x2c/0x40 > ? huge_pte_alloc+0x12c/0x1a0 > ? exc_invalid_op+0x33/0x50 > ? asm_exc_invalid_op+0x16/0x20 > ? __pfx_put_prev_task_idle+0x10/0x10 > ? huge_pte_alloc+0x12c/0x1a0 > hugetlb_fault+0x1a3/0x1120 > ? finish_task_switch+0xb3/0x2a0 > ? lock_is_held_type+0xdb/0x150 > handle_mm_fault+0xb8a/0xd40 > ? find_vma+0x5d/0xa0 > do_user_addr_fault+0x257/0x5d0 > exc_page_fault+0x7b/0x1f0 > asm_exc_page_fault+0x22/0x30 > > That happens because a BUG() statement in huge_pte_alloc() attempts to > check that a pte, if present, is a hugetlb pte, but it does so in a > non-lockless-safe manner that leads to a false BUG() report. > > We got here due to a couple of bugs, each of which by itself was not > quite enough to cause a problem: > > First of all, before commit c33c794828f2("mm: ptep_get() conversion"), > the BUG() statement in huge_pte_alloc() was itself fragile: it relied > upon compiler behavior to only read the pte once, despite using it twice > in the same conditional. > > Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that > delicate situation, by causing all direct pte reads to be done via > READ_ONCE(). And so READ_ONCE() got called twice within the same BUG() > conditional, leading to comparing (potentially, occasionally) different > versions of the pte, and thus to false BUG() reports. > > Fix this by taking a single snapshot of the pte before using it in the > BUG conditional. > > Now, that commit is only partially to blame here but, people doing > bisections will invariably land there, so this will help them find a fix > for a real crash. And also, the previous behavior was unlikely to ever > expose this bug--it was fragile, yet not actually broken. > > So that's why I chose this commit for the Fixes tag, rather than the > commit that created the original BUG() statement. > > Fixes: c33c794828f2 ("mm: ptep_get() conversion") > Cc: Adrian Hunter <adrian.hunter@intel.com> > Cc: Al Viro <viro@zeniv.linux.org.uk> > Cc: Alex Williamson <alex.williamson@redhat.com> > Cc: Alexander Potapenko <glider@google.com> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Andrey Konovalov <andreyknvl@gmail.com> > Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> > Cc: Christian Brauner <brauner@kernel.org> > Cc: Christoph Hellwig <hch@infradead.org> > Cc: Daniel Vetter <daniel@ffwll.ch> > Cc: Dave Airlie <airlied@gmail.com> > Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com> > Cc: Dmitry Vyukov <dvyukov@google.com> > Cc: Ian Rogers <irogers@google.com> > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Jiri Olsa <jolsa@kernel.org> > Cc: Johannes Weiner <hannes@cmpxchg.org> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Cc: Lorenzo Stoakes <lstoakes@gmail.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Miaohe Lin <linmiaohe@huawei.com> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Mike Kravetz <mike.kravetz@oracle.com> > Cc: Mike Rapoport (IBM) <rppt@kernel.org> > Cc: Muchun Song <muchun.song@linux.dev> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Naoya Horiguchi <naoya.horiguchi@nec.com> > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> > Cc: Pavel Tatashin <pasha.tatashin@soleen.com> > Cc: Roman Gushchin <roman.gushchin@linux.dev> > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: SeongJae Park <sj@kernel.org> > Cc: Shakeel Butt <shakeelb@google.com> > Cc: Uladzislau Rezki (Sony) <urezki@gmail.com> > Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> > Cc: Yu Zhao <yuzhao@google.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> > --- > mm/hugetlb.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) Thanks for catching this and the detailed explanation. Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index bce28cca73a1..73fbeb8f979f 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, pte = (pte_t *)pmd_alloc(mm, pud, addr); } } - BUG_ON(pte && pte_present(ptep_get(pte)) && !pte_huge(ptep_get(pte))); + + if (pte) { + pte_t pteval = ptep_get(pte); + + BUG_ON(pte_present(pteval) && !pte_huge(pteval)); + } return pte; }
The following crash happens for me when running the -mm selftests (below). Specifically, it happens while running the uffd-stress subtests: kernel BUG at mm/hugetlb.c:7249! invalid opcode: 0000 [#1] PREEMPT SMP NOPTI CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109 Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018 RIP: 0010:huge_pte_alloc+0x12c/0x1a0 ... Call Trace: <TASK> ? __die_body+0x63/0xb0 ? die+0x9f/0xc0 ? do_trap+0xab/0x180 ? huge_pte_alloc+0x12c/0x1a0 ? do_error_trap+0xc6/0x110 ? huge_pte_alloc+0x12c/0x1a0 ? handle_invalid_op+0x2c/0x40 ? huge_pte_alloc+0x12c/0x1a0 ? exc_invalid_op+0x33/0x50 ? asm_exc_invalid_op+0x16/0x20 ? __pfx_put_prev_task_idle+0x10/0x10 ? huge_pte_alloc+0x12c/0x1a0 hugetlb_fault+0x1a3/0x1120 ? finish_task_switch+0xb3/0x2a0 ? lock_is_held_type+0xdb/0x150 handle_mm_fault+0xb8a/0xd40 ? find_vma+0x5d/0xa0 do_user_addr_fault+0x257/0x5d0 exc_page_fault+0x7b/0x1f0 asm_exc_page_fault+0x22/0x30 That happens because a BUG() statement in huge_pte_alloc() attempts to check that a pte, if present, is a hugetlb pte, but it does so in a non-lockless-safe manner that leads to a false BUG() report. We got here due to a couple of bugs, each of which by itself was not quite enough to cause a problem: First of all, before commit c33c794828f2("mm: ptep_get() conversion"), the BUG() statement in huge_pte_alloc() was itself fragile: it relied upon compiler behavior to only read the pte once, despite using it twice in the same conditional. Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that delicate situation, by causing all direct pte reads to be done via READ_ONCE(). And so READ_ONCE() got called twice within the same BUG() conditional, leading to comparing (potentially, occasionally) different versions of the pte, and thus to false BUG() reports. Fix this by taking a single snapshot of the pte before using it in the BUG conditional. Now, that commit is only partially to blame here but, people doing bisections will invariably land there, so this will help them find a fix for a real crash. And also, the previous behavior was unlikely to ever expose this bug--it was fragile, yet not actually broken. So that's why I chose this commit for the Fixes tag, rather than the commit that created the original BUG() statement. Fixes: c33c794828f2 ("mm: ptep_get() conversion") Cc: Adrian Hunter <adrian.hunter@intel.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Alex Williamson <alex.williamson@redhat.com> Cc: Alexander Potapenko <glider@google.com> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrey Konovalov <andreyknvl@gmail.com> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> Cc: Christian Brauner <brauner@kernel.org> Cc: Christoph Hellwig <hch@infradead.org> Cc: Daniel Vetter <daniel@ffwll.ch> Cc: Dave Airlie <airlied@gmail.com> Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Ian Rogers <irogers@google.com> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Jiri Olsa <jolsa@kernel.org> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Lorenzo Stoakes <lstoakes@gmail.com> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Miaohe Lin <linmiaohe@huawei.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Mike Rapoport (IBM) <rppt@kernel.org> Cc: Muchun Song <muchun.song@linux.dev> Cc: Namhyung Kim <namhyung@kernel.org> Cc: Naoya Horiguchi <naoya.horiguchi@nec.com> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> Cc: Pavel Tatashin <pasha.tatashin@soleen.com> Cc: Roman Gushchin <roman.gushchin@linux.dev> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: SeongJae Park <sj@kernel.org> Cc: Shakeel Butt <shakeelb@google.com> Cc: Uladzislau Rezki (Sony) <urezki@gmail.com> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> Cc: Yu Zhao <yuzhao@google.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- mm/hugetlb.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) base-commit: bf1fa6f15553df04f2bdd06190ccd5f388ab0777