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From: Naoya Horiguchi To: John Hubbard , Miaohe Lin Cc: Matthew Wilcox , Christoph Hellwig , Andrew Morton , Jason Gunthorpe , William Kucharski , Naoya Horiguchi , linux-kernel@vger.kernel.org, Linux-MM Subject: Re: [PATCH v1] mm: recheck PageCompound() before printing out info of head page Message-ID: <20220416042536.GA1058054@u2004> References: <20220414235950.840409-1-naoya.horiguchi@linux.dev> <21807e0d-8bd1-7b1a-dd76-85a02267b424@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <21807e0d-8bd1-7b1a-dd76-85a02267b424@huawei.com> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Apr 16, 2022 at 09:56:41AM +0800, Miaohe Lin wrote: > On 2022/4/15 9:08, John Hubbard wrote: > > On 4/14/22 16:59, Naoya Horiguchi wrote: > >> From: Naoya Horiguchi > >> > >> The following VM_BUG_ON_FOLIO() is triggered when memory error event > >> happens on the (thp/folio) pages which are about to be freed: > >> > >>    [ 1160.232771] page:00000000b36a8a0f refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x16a000 > >>    [ 1160.236916] page:00000000b36a8a0f refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x16a000 > >>    [ 1160.240684] flags: 0x57ffffc0800000(hwpoison|node=1|zone=2|lastcpupid=0x1fffff) > >>    [ 1160.243458] raw: 0057ffffc0800000 dead000000000100 dead000000000122 0000000000000000 > >>    [ 1160.246268] raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000 > >>    [ 1160.249197] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio)) > >>    [ 1160.251815] ------------[ cut here ]------------ > >>    [ 1160.253438] kernel BUG at include/linux/mm.h:788! > >>    [ 1160.256162] invalid opcode: 0000 [#1] PREEMPT SMP PTI > >>    [ 1160.258172] CPU: 2 PID: 115368 Comm: mceinj.sh Tainted: G            E     5.18.0-rc1-v5.18-rc1-220404-2353-005-g83111+ #3 > >>    [ 1160.262049] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35 04/01/2014 > >>    [ 1160.265103] RIP: 0010:dump_page.cold+0x27e/0x2bd > >>    [ 1160.266757] Code: fe ff ff 48 c7 c6 81 f1 5a 98 e9 4c fe ff ff 48 c7 c6 a1 95 59 98 e9 40 fe ff ff 48 c7 c6 50 bf 5a 98 48 89 ef e8 9d 04 6d ff <0f> 0b 41 f7 c4 ff 0f 00 00 0f 85 9f fd ff ff 49 8b 04 24 a9 00 00 > >>    [ 1160.273180] RSP: 0018:ffffaa2c4d59fd18 EFLAGS: 00010292 > >>    [ 1160.274969] RAX: 000000000000003e RBX: 0000000000000001 RCX: 0000000000000000 > >>    [ 1160.277263] RDX: 0000000000000001 RSI: ffffffff985995a1 RDI: 00000000ffffffff > >>    [ 1160.279571] RBP: ffffdc9c45a80000 R08: 0000000000000000 R09: 00000000ffffdfff > >>    [ 1160.281794] R10: ffffaa2c4d59fb08 R11: ffffffff98940d08 R12: ffffdc9c45a80000 > >>    [ 1160.283920] R13: ffffffff985b6f94 R14: 0000000000000000 R15: ffffdc9c45a80000 > >>    [ 1160.286641] FS:  00007eff54ce1740(0000) GS:ffff99c67bd00000(0000) knlGS:0000000000000000 > >>    [ 1160.289498] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > >>    [ 1160.291106] CR2: 00005628381a5f68 CR3: 0000000104712003 CR4: 0000000000170ee0 > >>    [ 1160.293031] Call Trace: > >>    [ 1160.293724]  > >>    [ 1160.294334]  get_hwpoison_page+0x47d/0x570 > >>    [ 1160.295474]  memory_failure+0x106/0xaa0 > >>    [ 1160.296474]  ? security_capable+0x36/0x50 > >>    [ 1160.297524]  hard_offline_page_store+0x43/0x80 > >>    [ 1160.298684]  kernfs_fop_write_iter+0x11c/0x1b0 > >>    [ 1160.299829]  new_sync_write+0xf9/0x160 > >>    [ 1160.300810]  vfs_write+0x209/0x290 > >>    [ 1160.301835]  ksys_write+0x4f/0xc0 > >>    [ 1160.302718]  do_syscall_64+0x3b/0x90 > >>    [ 1160.303664]  entry_SYSCALL_64_after_hwframe+0x44/0xae > >>    [ 1160.304981] RIP: 0033:0x7eff54b018b7 > >> > >> As shown in the RIP address, this VM_BUG_ON in folio_entire_mapcount() is > >> called from dump_page("hwpoison: unhandlable page") in get_any_page(). > >> The below explains the mechanism of the race: > >> > >>    CPU 0                                       CPU 1 > >> > >>      memory_failure > >>        get_hwpoison_page > >>          get_any_page > >>            dump_page > >>              compound = PageCompound > >>                                                  free_pages_prepare > >>                                                    page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP > >>              folio_entire_mapcount > >>                VM_BUG_ON_FOLIO(!folio_test_large(folio)) > >> > >> So in order to avoid the BUG_ON, recheck PageCompound() before __dump_page() > >> prints out information about the head page.  Precisely saying, this does > >> not close up the race, but mitigates the impact to just inaccurate info, > >> which should be acceptable. > > > > Actually, I don't think it really closes the race even to that extent, > > because a machine check (MCE, on x86) could come in at any point, so > > rechecking (without locks) won't really completely fix such a bug, right? > > It might also be unsafe to do dump_mapping or dump_page_owner when page could > be freed at any time or we might come across some use-after-free issues? OK, so memory_failure() simply shouldn't call dump_page() before taking page refcnt. > > > > >> > >> Fixes: 74e8ee4708a8 ("mm: Turn head_compound_mapcount() into folio_entire_mapcount()") > >> Signed-off-by: Naoya Horiguchi > >> --- > >> I'm actually not sure how widely the race condition could affect. > >> If we can consider this as a bug specific to memory_failure(), simply > >> replacing the dump_page() with simple printk() might be a proper fix > > > > I think we should have separate, limited, special case functions that > > are safe for use by exotic things like the MCE handler. And so just > > doing a printk there seems better to me. Thanks, so now I lean to the printk-approach. > > > >> (I confirmed that that fix also works).  But if any other dump_page()'s > >> caller could race with freeing thp/folio, the more generic fix > >> (I'm suggesting in this patch) would be desirable. > > > > *If* there are other races, they should be addressed separately, IMHO. > > Because MCE should be a special case. I agree. Thanks, Naoya Horiguchi > > > >> --- > >>   mm/debug.c | 2 +- > >>   1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/mm/debug.c b/mm/debug.c > >> index bef329bf28f0..fce63b200bea 100644 > >> --- a/mm/debug.c > >> +++ b/mm/debug.c > >> @@ -93,7 +93,7 @@ static void __dump_page(struct page *page) > >>       pr_warn("page:%p refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n", > >>               page, page_ref_count(head), mapcount, mapping, > >>               page_to_pgoff(page), page_to_pfn(page)); > >> -    if (compound) { > >> +    if (compound && PageCompound(page)) { > > > > If people decide to keep this approach, then I'd go with something > > that makes it clear, like: > > > >     /* Re-check, in a semi-doomed attempt to avoid racing with MCE: */ > >     compound = PageCompound(page); > > Agree. A comment will be needed or the code might look obscure. > > Thanks! > > > > > :) > > > > thanks, >