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Wed, 19 Feb 2025 12:54:01 -0800 (PST) MIME-Version: 1.0 References: <20250219112519.92853-1-21cnbao@gmail.com> In-Reply-To: From: Lokesh Gidra Date: Wed, 19 Feb 2025 12:53:50 -0800 X-Gm-Features: AWEUYZmVfXW8ncrvvsoJTz9S1RYAbffZUNrLLxR_kGimZu4DtDFyMcCgGExpsq4 Message-ID: Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Barry Song <21cnbao@gmail.com> Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, zhengtangquan@oppo.com, Barry Song , Andrea Arcangeli , Suren Baghdasaryan , Al Viro , Axel Rasmussen , Brian Geffon , Christian Brauner , David Hildenbrand , Hugh Dickins , Jann Horn , Kalesh Singh , "Liam R . Howlett" , Matthew Wilcox , Michal Hocko , Mike Rapoport , Nicolas Geoffray , Peter Xu , Ryan Roberts , Shuah Khan , ZhangPeng Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 05A58C0004 X-Stat-Signature: g4hi7kdgakhackn58szehoah5bk9borg X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1739998442-534563 X-HE-Meta: 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 E4oL/oug 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 19, 2025 at 12:45=E2=80=AFPM Barry Song <21cnbao@gmail.com> wro= te: > > On Thu, Feb 20, 2025 at 7:40=E2=80=AFAM Lokesh Gidra wrote: > > > > On Wed, Feb 19, 2025 at 3:25=E2=80=AFAM Barry Song <21cnbao@gmail.com> = wrote: > > > > > > From: Barry Song > > > > > > userfaultfd_move() checks whether the PTE entry is present or a > > > swap entry. > > > > > > - If the PTE entry is present, move_present_pte() handles folio > > > migration by setting: > > > > > > src_folio->index =3D linear_page_index(dst_vma, dst_addr); > > > > > > - If the PTE entry is a swap entry, move_swap_pte() simply copies > > > the PTE to the new dst_addr. > > > > > > This approach is incorrect because even if the PTE is a swap > > > entry, it can still reference a folio that remains in the swap > > > cache. > > > > > > If do_swap_page() is triggered, it may locate the folio in the > > > swap cache. However, during add_rmap operations, a kernel panic > > > can occur due to: > > > page_pgoff(folio, page) !=3D linear_page_index(vma, address) > > > > > > $./a.out > /dev/null > > > [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c i= ndex:0xffffaf150 pfn:0x4667c > > > [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_ma= pped:1 pincount:0 > > > [ 13.337716] memcg:ffff00000405f000 > > > [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|ow= ner_priv_1|head|swapbacked|node=3D0|zone=3D0|lastcpupid=3D0xffff) > > > [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b53= 8 ffff000006260361 > > > [ 13.338831] raw: 0000000ffffaf150 0000000000004000 000000060000000= 0 ffff00000405f000 > > > [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b5= 38 ffff000006260361 > > > [ 13.339204] head: 0000000ffffaf150 0000000000004000 00000006000000= 00 ffff00000405f000 > > > [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff000000= 00 0000000000000001 > > > [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffff= ff 0000000000000000 > > > [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, = page) !=3D linear_page_index(vma, address)) > > > [ 13.340190] ------------[ cut here ]------------ > > > [ 13.340316] kernel BUG at mm/rmap.c:1380! > > > [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREE= MPT SMP > > > [ 13.340969] Modules linked in: > > > [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-= rc3-gcf42737e247a-dirty #299 > > > [ 13.341470] Hardware name: linux,dummy-virt (DT) > > > [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS = BTYPE=3D--) > > > [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 > > > [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 > > > [ 13.342018] sp : ffff80008752bb20 > > > [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 00000= 00000000001 > > > [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 00000= 00000000001 > > > [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffff= dffc0199f00 > > > [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000= 000ffffffff > > > [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 66286= 6666f67705f > > > [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff8= 00083728ab0 > > > [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff8= 0008011bc40 > > > [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8= 000829eebf8 > > > [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 00000= 00000000000 > > > [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 00000= 0000000005f > > > [ 13.343876] Call trace: > > > [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) > > > [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 > > > [ 13.344333] do_swap_page+0x1060/0x1400 > > > [ 13.344417] __handle_mm_fault+0x61c/0xbc8 > > > [ 13.344504] handle_mm_fault+0xd8/0x2e8 > > > [ 13.344586] do_page_fault+0x20c/0x770 > > > [ 13.344673] do_translation_fault+0xb4/0xf0 > > > [ 13.344759] do_mem_abort+0x48/0xa0 > > > [ 13.344842] el0_da+0x58/0x130 > > > [ 13.344914] el0t_64_sync_handler+0xc4/0x138 > > > [ 13.345002] el0t_64_sync+0x1ac/0x1b0 > > > [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) > > > [ 13.345504] ---[ end trace 0000000000000000 ]--- > > > [ 13.345715] note: a.out[107] exited with irqs disabled > > > [ 13.345954] note: a.out[107] exited with preempt_count 2 > > > > > > Fully fixing it would be quite complex, requiring similar handling > > > of folios as done in move_present_pte. For now, a quick solution > > > is to return -EBUSY. > > > I'd like to see others' opinions on whether a full fix is worth > > > pursuing. > > > > > > > Thanks a lot for finding this. > > > > As a user of MOVE ioctl (in Android GC) I strongly urge you to fix > > this properly. Because this is not going to be a rare occurrence in > > the case of Android. And when -EBUSY is returned, all that userspace > > can do is touch the page, which also does not guarantee that a > > subsequent retry of the ioctl will succeed. > > Not trying to push this idea, but I=E2=80=99m curious if it's feasible: > > If UFFDIO_MOVE fails, could userspace fall back to UFFDIO_COPY? It's possible! But it wouldn't be rare to find such pages and falling back to COPY so many times would mellow down the benefits of using MOVE quite a bit. > > I=E2=80=99m still trying to wrap my head around a few things, particularl= y > what exactly UFFDIO_MOVE is doing with mTHP, as I mentioned in my > reply to Suren and you in another email: > > https://lore.kernel.org/linux-mm/CAGsJ_4yx1=3DjaQmDG_9rMqHFFkoXqMJw941eYv= tby28OqDq+S7g@mail.gmail.com/ > > > > > > > For anyone interested in reproducing it, the a.out test program is > > > as below, > > > > > > #define _GNU_SOURCE > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > #include > > > > > > #define PAGE_SIZE 4096 > > > #define REGION_SIZE (512 * 1024) > > > > > > #ifndef UFFDIO_MOVE > > > struct uffdio_move { > > > __u64 dst; > > > __u64 src; > > > __u64 len; > > > #define UFFDIO_MOVE_MODE_DONTWAKE ((__u64)1<<0) > > > #define UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES ((__u64)1<<1) > > > __u64 mode; > > > __s64 move; > > > }; > > > #define _UFFDIO_MOVE (0x05) > > > #define UFFDIO_MOVE _IOWR(UFFDIO, _UFFDIO_MOVE, struct uffdio_move= ) > > > #endif > > > > > > void *src, *dst; > > > int uffd; > > > > > > void *madvise_thread(void *arg) { > > > if (madvise(src, REGION_SIZE, MADV_PAGEOUT) =3D=3D -1) { > > > perror("madvise MADV_PAGEOUT"); > > > } > > > return NULL; > > > } > > > > > > void *fault_handler_thread(void *arg) { > > > struct uffd_msg msg; > > > struct uffdio_move move; > > > struct pollfd pollfd =3D { .fd =3D uffd, .events =3D POLLIN }; > > > > > > pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL); > > > pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL); > > > > > > while (1) { > > > if (poll(&pollfd, 1, -1) =3D=3D -1) { > > > perror("poll"); > > > exit(EXIT_FAILURE); > > > } > > > > > > if (read(uffd, &msg, sizeof(msg)) <=3D 0) { > > > perror("read"); > > > exit(EXIT_FAILURE); > > > } > > > > > > if (msg.event !=3D UFFD_EVENT_PAGEFAULT) { > > > fprintf(stderr, "Unexpected event\n"); > > > exit(EXIT_FAILURE); > > > } > > > > > > move.src =3D (unsigned long)src + (msg.arg.pagefault.address= - (unsigned long)dst); > > > move.dst =3D msg.arg.pagefault.address & ~(PAGE_SIZE - 1); > > > move.len =3D PAGE_SIZE; > > > move.mode =3D 0; > > > > > > if (ioctl(uffd, UFFDIO_MOVE, &move) =3D=3D -1) { > > > perror("UFFDIO_MOVE"); > > > exit(EXIT_FAILURE); > > > } > > > } > > > return NULL; > > > } > > > > > > int main() { > > > again: > > > pthread_t thr, madv_thr; > > > struct uffdio_api uffdio_api =3D { .api =3D UFFD_API, .features = =3D 0 }; > > > struct uffdio_register uffdio_register; > > > > > > src =3D mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIV= ATE | MAP_ANONYMOUS, -1, 0); > > > if (src =3D=3D MAP_FAILED) { > > > perror("mmap src"); > > > exit(EXIT_FAILURE); > > > } > > > memset(src, 1, REGION_SIZE); > > > > > > dst =3D mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIV= ATE | MAP_ANONYMOUS, -1, 0); > > > if (dst =3D=3D MAP_FAILED) { > > > perror("mmap dst"); > > > exit(EXIT_FAILURE); > > > } > > > > > > uffd =3D syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK); > > > if (uffd =3D=3D -1) { > > > perror("userfaultfd"); > > > exit(EXIT_FAILURE); > > > } > > > > > > if (ioctl(uffd, UFFDIO_API, &uffdio_api) =3D=3D -1) { > > > perror("UFFDIO_API"); > > > exit(EXIT_FAILURE); > > > } > > > > > > uffdio_register.range.start =3D (unsigned long)dst; > > > uffdio_register.range.len =3D REGION_SIZE; > > > uffdio_register.mode =3D UFFDIO_REGISTER_MODE_MISSING; > > > > > > if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) =3D=3D -1) { > > > perror("UFFDIO_REGISTER"); > > > exit(EXIT_FAILURE); > > > } > > > > > > if (pthread_create(&madv_thr, NULL, madvise_thread, NULL) !=3D 0= ) { > > > perror("pthread_create madvise_thread"); > > > exit(EXIT_FAILURE); > > > } > > > > > > if (pthread_create(&thr, NULL, fault_handler_thread, NULL) !=3D = 0) { > > > perror("pthread_create fault_handler_thread"); > > > exit(EXIT_FAILURE); > > > } > > > > > > for (size_t i =3D 0; i < REGION_SIZE; i +=3D PAGE_SIZE) { > > > char val =3D ((char *)dst)[i]; > > > printf("Accessing dst at offset %zu, value: %d\n", i, val); > > > } > > > > > > pthread_join(madv_thr, NULL); > > > pthread_cancel(thr); > > > pthread_join(thr, NULL); > > > > > > munmap(src, REGION_SIZE); > > > munmap(dst, REGION_SIZE); > > > close(uffd); > > > goto again; > > > return 0; > > > } > > > > > > As long as you enable mTHP (which likely increases the residency > > > time of swapcache), you can reproduce the issue within a few > > > seconds. But I guess the same race condition also exists with > > > small folios. > > > > > > Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI") > > > Cc: Andrea Arcangeli > > > Cc: Suren Baghdasaryan > > > Cc: Al Viro > > > Cc: Axel Rasmussen > > > Cc: Brian Geffon > > > Cc: Christian Brauner > > > Cc: David Hildenbrand > > > Cc: Hugh Dickins > > > Cc: Jann Horn > > > Cc: Kalesh Singh > > > Cc: Liam R. Howlett > > > Cc: Lokesh Gidra > > > Cc: Matthew Wilcox (Oracle) > > > Cc: Michal Hocko > > > Cc: Mike Rapoport (IBM) > > > Cc: Nicolas Geoffray > > > Cc: Peter Xu > > > Cc: Ryan Roberts > > > Cc: Shuah Khan > > > Cc: ZhangPeng > > > Signed-off-by: Barry Song > > > --- > > > mm/userfaultfd.c | 11 +++++++++++ > > > 1 file changed, 11 insertions(+) > > > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > > index 867898c4e30b..34cf1c8c725d 100644 > > > --- a/mm/userfaultfd.c > > > +++ b/mm/userfaultfd.c > > > @@ -18,6 +18,7 @@ > > > #include > > > #include > > > #include "internal.h" > > > +#include "swap.h" > > > > > > static __always_inline > > > bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long = dst_end) > > > @@ -1079,9 +1080,19 @@ static int move_swap_pte(struct mm_struct *mm, > > > pmd_t *dst_pmd, pmd_t dst_pmdval, > > > spinlock_t *dst_ptl, spinlock_t *src_ptl) > > > { > > > + struct folio *folio; > > > + swp_entry_t entry; > > > + > > > if (!pte_swp_exclusive(orig_src_pte)) > > > return -EBUSY; > > > > > > + entry =3D pte_to_swp_entry(orig_src_pte); > > > + folio =3D filemap_get_folio(swap_address_space(entry), swap_c= ache_index(entry)); > > > + if (!IS_ERR(folio)) { > > > + folio_put(folio); > > > + return -EBUSY; > > > + } > > > + > > > double_pt_lock(dst_ptl, src_ptl); > > > > > > if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig= _src_pte, > > > -- > > > 2.39.3 (Apple Git-146) > > > > > > > Thanks > Barry