On Tue, 6 Jun 2023 19:36:13 -0400 Rafael Aquini <aquini@xxxxxxxxxx> wrote:
> When commit 19343b5bdd16 ("mm/page-writeback: introduce tracepoint for
> wait_on_page_writeback()") repurposed the writeback_dirty_page trace event
> as a template to create its new wait_on_page_writeback trace event, it
> ended up opening a window to NULL pointer dereference crashes due to
> the (infrequent) occurrence of a race where an access to a page in the
> swap-cache happens concurrently with the moment this page is being
> written to disk and the tracepoint is enabled:
I don't see what the race is, or why a race is involved.
> BUG: kernel NULL pointer dereference, address: 0000000000000040
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 800000010ec0a067 P4D 800000010ec0a067 PUD 102353067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 1 PID: 1320 Comm: shmem-worker Kdump: loaded Not tainted 6.4.0-rc5+ #13
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20230301gitf80f052277c8-1.fc37 03/01/2023
> RIP: 0010:trace_event_raw_event_writeback_folio_template+0x76/0xf0
> Code: 4d 85 e4 74 5c 49 8b 3c 24 e8 06 98 ee ff 48 89 c7 e8 9e 8b ee ff ba 20 00 00 00 48 89 ef 48 89 c6 e8 fe d4 1a 00 49 8b 04 24 <48> 8b 40 40 48 89 43 28 49 8b 45 20 48 89 e7 48 89 43 30 e8 a2 4d
> RSP: 0000:ffffaad580b6fb60 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff90e38035c01c RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff90e38035c044
> RBP: ffff90e38035c024 R08: 0000000000000002 R09: 0000000000000006
> R10: ffff90e38035c02e R11: 0000000000000020 R12: ffff90e380bac000
> R13: ffffe3a7456d9200 R14: 0000000000001b81 R15: ffffe3a7456d9200
> FS: 00007f2e4e8a15c0(0000) GS:ffff90e3fbc80000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000040 CR3: 00000001150c6003 CR4: 0000000000170ee0
> Call Trace:
> <TASK>
> ? __die+0x20/0x70
> ? page_fault_oops+0x76/0x170
> ? kernelmode_fixup_or_oops+0x84/0x110
> ? exc_page_fault+0x65/0x150
> ? asm_exc_page_fault+0x22/0x30
> ? trace_event_raw_event_writeback_folio_template+0x76/0xf0
> folio_wait_writeback+0x6b/0x80
> shmem_swapin_folio+0x24a/0x500
shmem_swapin_folio->folio_wait_writeback will always pass in a page
which has ->mapping==NULL, won't it? So why doesn't it crash every
time?
> ? filemap_get_entry+0xe3/0x140
> shmem_get_folio_gfp+0x36e/0x7c0
> ? find_busiest_group+0x43/0x1a0
> shmem_fault+0x76/0x2a0
> ? __update_load_avg_cfs_rq+0x281/0x2f0
> __do_fault+0x33/0x130
> do_read_fault+0x118/0x160
> do_pte_missing+0x1ed/0x2a0
> __handle_mm_fault+0x566/0x630
> handle_mm_fault+0x91/0x210
> do_user_addr_fault+0x22c/0x740
> exc_page_fault+0x65/0x150
> asm_exc_page_fault+0x22/0x30
>
> This problem arises from the fact that the repurposed writeback_dirty_page
> trace event code was written assuming that every pointer to mapping
> (struct address_space) would come from a file-mapped page-cache object,
> thus mapping->host would always be populated, and that was a valid case
> before commit 19343b5bdd16. The swap-cache address space (swapper_spaces),
> however, doesn't populate its ->host (struct inode) pointer, thus leading
> to the crashes in the corner-case aforementioned.
>
> commit 19343b5bdd16 ended up breaking the assignment of __entry->name and
> __entry->ino for the wait_on_page_writeback tracepoint -- both dependent
> on mapping->host carrying a pointer to a valid inode. The assignment of
> __entry->name was fixed by commit 68f23b89067f ("memcg: fix a crash in
> wb_workfn when a device disappears"), and this commit fixes the remaining
> case, for __entry->ino.
