Hi Ryan, On Tue, Mar 05, 2024 at 03:13:49PM +0000, Ryan Roberts wrote: > There was previously a theoretical window where swapoff() could run and > teardown a swap_info_struct while a call to free_swap_and_cache() was > running in another thread. This could cause, amongst other bad > possibilities, swap_page_trans_huge_swapped() (called by > free_swap_and_cache()) to access the freed memory for swap_map. > > This is a theoretical problem and I haven't been able to provoke it from > a test case. But there has been agreement based on code review that this > is possible (see link below). > > Fix it by using get_swap_device()/put_swap_device(), which will stall > swapoff(). There was an extra check in _swap_info_get() to confirm that > the swap entry was valid. This wasn't present in get_swap_device() so > I've added it. I couldn't find any existing get_swap_device() call sites > where this extra check would cause any false alarms. Unfortunately, I found one, testing current mm-everything: [ 189.420777] get_swap_device: Unused swap offset entry 000641ae [ 189.426648] ------------[ cut here ]------------ [ 189.431290] WARNING: CPU: 3 PID: 369 at mm/swapfile.c:1301 get_swap_device+0x2da/0x3f0 [ 189.439242] CPU: 3 PID: 369 Comm: cachish Not tainted 6.8.0-rc5-00527-g19d98776f227-dirty #30 [ 189.447791] Hardware name: Micro-Star International Co., Ltd. MS-7B98/Z390-A PRO (MS-7B98), BIOS 1.80 12/25/2019 [ 189.457998] RIP: 0010:get_swap_device+0x2da/0x3f0 [ 189.462721] Code: a8 03 75 2a 65 48 ff 08 e9 36 ff ff ff 4c 89 e9 48 c7 c2 40 fd 91 83 48 c7 c6 c0 f9 91 83 48 c7 c7 60 ee 91 83 e8 26 2f af ff <0f> 0b eb af 4c 8d 6b 08 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 [ 189.481497] RSP: 0000:ffffc90000cff8a8 EFLAGS: 00010282 [ 189.486760] RAX: 0000000000000032 RBX: ffff8881262eee00 RCX: 0000000000000000 [ 189.493909] RDX: 0000000000000001 RSI: ffffffff83a1e620 RDI: 0000000000000001 [ 189.501054] RBP: 1ffff9200019ff15 R08: 0000000000000001 R09: fffff5200019fee1 [ 189.508202] R10: ffffc90000cff70f R11: 0000000000000001 R12: ffffc900018d51ae [ 189.515346] R13: 00000000000641ae R14: 0000000000000000 R15: 00000000000641af [ 189.522494] FS: 00007f7120263680(0000) GS:ffff88841c380000(0000) knlGS:0000000000000000 [ 189.530591] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 189.536373] CR2: 00007f6e659a2ea3 CR3: 0000000046860004 CR4: 00000000003706f0 [ 189.543516] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 189.550661] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 189.557811] Call Trace: [ 189.560276] <TASK> [ 189.562393] ? __warn+0xc4/0x250 [ 189.565647] ? get_swap_device+0x2da/0x3f0 [ 189.569761] ? report_bug+0x348/0x440 [ 189.573444] ? handle_bug+0x6d/0x90 [ 189.576951] ? exc_invalid_op+0x13/0x40 [ 189.580810] ? asm_exc_invalid_op+0x16/0x20 [ 189.585019] ? get_swap_device+0x2da/0x3f0 [ 189.589142] ? get_swap_device+0x2da/0x3f0 [ 189.593255] ? __pfx_get_swap_device+0x10/0x10 [ 189.597717] __read_swap_cache_async+0x9f/0x630 [ 189.602281] ? __pfx___read_swap_cache_async+0x10/0x10 [ 189.607439] ? __mod_memcg_lruvec_state+0x238/0x4f0 [ 189.612344] ? __pfx_swp_swap_info+0x10/0x10 [ 189.616652] swap_cluster_readahead+0x2cd/0x510 [ 189.621206] ? __pfx_swap_cluster_readahead+0x10/0x10 [ 189.626279] ? swap_cache_get_folio+0xcd/0x360 [ 189.630760] ? __count_memcg_events+0x10a/0x370 [ 189.635318] shmem_swapin_folio+0x2f2/0xc60 [ 189.639525] ? __pfx__raw_spin_lock+0x10/0x10 [ 189.643908] ? __pte_offset_map+0x19/0x1d0 [ 189.648024] shmem_get_folio_gfp+0x307/0xe30 [ 189.652323] ? __schedule+0x9f0/0x1fe0 [ 189.656096] ? __pfx_shmem_get_folio_gfp+0x10/0x10 [ 189.660923] ? filemap_map_pages+0x999/0xe60 [ 189.665211] shmem_fault+0x1d9/0x810 [ 189.668834] ? __pfx_shmem_fault+0x10/0x10 [ 189.672954] ? __pfx_filemap_map_pages+0x10/0x10 [ 189.677590] __do_fault+0xed/0x390 [ 189.681012] __handle_mm_fault+0x1ba1/0x2e80 [ 189.685297] ? __pfx___handle_mm_fault+0x10/0x10 [ 189.689933] ? __pfx_down_read_trylock+0x10/0x10 [ 189.694570] ? __pfx_hrtimer_nanosleep+0x10/0x10 [ 189.699215] handle_mm_fault+0xe0/0x560 [ 189.703074] ? __pfx_restore_fpregs_from_fpstate+0x10/0x10 [ 189.708620] do_user_addr_fault+0x2ba/0x9d0 [ 189.712828] exc_page_fault+0x54/0x90 [ 189.716508] asm_exc_page_fault+0x22/0x30 [ 189.720535] RIP: 0033:0x5640dc2d72b5 [ 189.724131] Code: 98 48 ba 00 00 00 00 03 00 00 00 48 89 d6 48 2b 75 a0 ba 00 00 00 00 48 f7 f6 48 89 d1 48 89 ca 48 8b 45 a0 48 01 d0 48 01 d8 <0f> b6 00 bf e8 03 00 00 e8 1e fe ff ff 48 83 45 a8 01 48 8d 45 d0 [ 189.742922] RSP: 002b:00007ffc227e3f60 EFLAGS: 00010206 [ 189.748165] RAX: 00007f6e659a2ea3 RBX: 00007f6e2007e000 RCX: 0000000045924ea3 [ 189.755311] RDX: 0000000045924ea3 RSI: 0000000300000000 RDI: 00007f71202586a0 [ 189.762483] RBP: 00007ffc227e3fe0 R08: 00007f7120258074 R09: 00007f71202580a0 [ 189.769633] R10: 0000000000019458 R11: 00000000008aa400 R12: 0000000000000000 [ 189.776781] R13: 00007ffc227e4128 R14: 00007f712029d000 R15: 00005640dc2d9dd8 [ 189.783928] </TASK> [ 189.786126] ---[ end trace 0000000000000000 ]--- [ 285.827888] get_swap_device: Unused swap offset entry 0018403f [ 320.699306] get_swap_device: Unused swap offset entry 000b001b [ 354.031339] get_swap_device: Unused swap offset entry 000681a9 [ 364.958435] get_swap_device: Unused swap offset entry 001f4055 [ 364.976235] get_swap_device: Unused swap offset entry 001f4057 [ 365.530174] get_swap_device: Unused swap offset entry 000d415c [ 394.223792] get_swap_device: Unused swap offset entry 001540d0 [ 394.317299] get_swap_device: Unused swap offset entry 000341d9 [ 394.341727] get_swap_device: Unused swap offset entry 0006c07e [ 396.062365] get_swap_device: Unused swap offset entry 000541a4 [ 396.068262] get_swap_device: Unused swap offset entry 000541a7 [ 402.629551] get_swap_device: Unused swap offset entry 00294021 [ 436.740622] get_swap_device: Unused swap offset entry 00334155 [ 436.758527] get_swap_device: Unused swap offset entry 001b417c swap_cluster_readahead() calls __read_swap_cache_async() on a range of made-up swap entries around the faulting slot. The device and the range (si->max) are valid, but the specific entry might not be in use. __read_swap_cache_async() instead relies on swapcache_prepare() returning -ENOENT to catch this and skip gracefully. Confirmed that reverting the patch makes the warnings go away.
