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From:   Rongwei Wang <rongwei.wang@linux.alibaba.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     akpm@linux-foundation.org, willy@infradead.org, song@kernel.org,
        william.kucharski@oracle.com, hughd@google.com, shy828301@gmail.com
Subject: [PATCH v4 1/2] mm, thp: lock filemap when truncating page cache
Date:   Mon, 11 Oct 2021 10:22:40 +0800
Message-Id: <20211011022241.97072-2-rongwei.wang@linux.alibaba.com>
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 <20211011022241.97072-1-rongwei.wang@linux.alibaba.com>
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Transparent huge page has supported read-only non-shmem files. The file-
backed THP is collapsed by khugepaged and truncated when written (for
shared libraries).

However, there is a race when multiple writers truncate the same page
cache concurrently.

In that case, subpage(s) of file THP can be revealed by find_get_entry
in truncate_inode_pages_range, which will trigger PageTail BUG_ON in
truncate_inode_page, as follows.

page:000000009e420ff2 refcount:1 mapcount:0 mapping:0000000000000000 index:0x7ff pfn:0x50c3ff
head:0000000075ff816d order:9 compound_mapcount:0 compound_pincount:0
flags: 0x37fffe0000010815(locked|uptodate|lru|arch_1|head)
raw: 37fffe0000000000 fffffe0013108001 dead000000000122 dead000000000400
raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
head: 37fffe0000010815 fffffe001066bd48 ffff000404183c20 0000000000000000
head: 0000000000000600 0000000000000000 00000001ffffffff ffff000c0345a000
page dumped because: VM_BUG_ON_PAGE(PageTail(page))
------------[ cut here ]------------
kernel BUG at mm/truncate.c:213!
Internal error: Oops - BUG: 0 [#1] SMP
Modules linked in: xfs(E) libcrc32c(E) rfkill(E) ...
CPU: 14 PID: 11394 Comm: check_madvise_d Kdump: ...
Hardware name: ECS, BIOS 0.0.0 02/06/2015
pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
pc : truncate_inode_page+0x64/0x70
lr : truncate_inode_page+0x64/0x70
sp : ffff80001b60b900
x29: ffff80001b60b900 x28: 00000000000007ff
x27: ffff80001b60b9a0 x26: 0000000000000000
x25: 000000000000000f x24: ffff80001b60b9a0
x23: ffff80001b60ba18 x22: ffff0001e0999ea8
x21: ffff0000c21db300 x20: ffffffffffffffff
x19: fffffe001310ffc0 x18: 0000000000000020
x17: 0000000000000000 x16: 0000000000000000
x15: ffff0000c21db960 x14: 3030306666666620
x13: 6666666666666666 x12: 3130303030303030
x11: ffff8000117b69b8 x10: 00000000ffff8000
x9 : ffff80001012690c x8 : 0000000000000000
x7 : ffff8000114f69b8 x6 : 0000000000017ffd
x5 : ffff0007fffbcbc8 x4 : ffff80001b60b5c0
x3 : 0000000000000001 x2 : 0000000000000000
x1 : 0000000000000000 x0 : 0000000000000000
Call trace:
 truncate_inode_page+0x64/0x70
 truncate_inode_pages_range+0x550/0x7e4
 truncate_pagecache+0x58/0x80
 do_dentry_open+0x1e4/0x3c0
 vfs_open+0x38/0x44
 do_open+0x1f0/0x310
 path_openat+0x114/0x1dc
 do_filp_open+0x84/0x134
 do_sys_openat2+0xbc/0x164
 __arm64_sys_openat+0x74/0xc0
 el0_svc_common.constprop.0+0x88/0x220
 do_el0_svc+0x30/0xa0
 el0_svc+0x20/0x30
 el0_sync_handler+0x1a4/0x1b0
 el0_sync+0x180/0x1c0
Code: aa0103e0 900061e1 910ec021 9400d300 (d4210000)
---[ end trace f70cdb42cb7c2d42 ]---
Kernel panic - not syncing: Oops - BUG: Fatal exception

This patch mainly to lock filemap when one enter truncate_pagecache(),
avoiding truncating the same page cache concurrently.

Fixes: eb6ecbed0aa2 ("mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs")
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Song Liu <song@kernel.org>
Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
---
 fs/open.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/open.c b/fs/open.c
index daa324606a41..9ec3cfca3b1a 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -856,8 +856,11 @@ static int do_dentry_open(struct file *f,
 		 * of THPs into the page cache will fail.
 		 */
 		smp_mb();
-		if (filemap_nr_thps(inode->i_mapping))
+		if (filemap_nr_thps(inode->i_mapping)) {
+			filemap_invalidate_lock(inode->i_mapping);
 			truncate_pagecache(inode, 0);
+			filemap_invalidate_unlock(inode->i_mapping);
+		}
 	}
 
 	return 0;
-- 
2.27.0