[PATCH] mm: get rid of WARN if failed to cow user pages

[Murphy Zhou <jencce.kernel@gmail.com>]

By running xfstests with fsdax enabled, generic/437 always hits this
warning[1] since this commit:

commit 83d116c53058d505ddef051e90ab27f57015b025
Author: Jia He <justin.he@arm.com>
Date:   Fri Oct 11 22:09:39 2019 +0800

    mm: fix double page fault on arm64 if PTE_AF is cleared

Looking at the test program[2] generic/437 uses, it's pretty easy
to hit this warning. Remove this WARN as it seems not necessary.

[2] https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/src/t_mmap_cow_race.c
[1] warning message:
-----------------------------------------------------------------------
[   97.344077] WARNING: CPU: 0 PID: 2486 at mm/memory.c:2281 wp_page_copy+0x687/0x6e0
[   97.348354] Modules linked in: nf_tables nfnetlink rfkill sunrpc snd_hda_codec_generic ledtrig_audio qxl snd_hda_intel snd_intel_dspcfg drm_ttm_helper snd_hda_codec ttm snd_hda_core drm_kms_helper snd_hwdep snd_seq syscopyarea sysfillrect sysimgblt snd_seq_device fb_sys_fops snd_pcm drm snd_timer crc32_pclmul snd soundcore dax_pmem_compat i2c_piix4 device_dax virtio_balloon pcspkr joydev dax_pmem_core ip_tables xfs libcrc32c crct10dif_pclmul crc32c_intel sd_mod sg ata_generic 8139too ata_piix libata ghash_clmulni_intel 8139cp virtio_console serio_raw nd_pmem mii dm_mirror dm_region_hash dm_log dm_mod
[   97.382176] CPU: 0 PID: 2486 Comm: t_mmap_cow_race Tainted: G        W         5.5.0-rc3-v5.5-rc3 #1
[   97.387804] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[   97.392228] RIP: 0010:wp_page_copy+0x687/0x6e0
[   97.396572] Code: 95 f5 00 48 81 e6 00 f0 ff ff ba 00 10 00 00 49 c1 ff 06 49 c1 e7 0c 4c 03 3d 35 95 f5 00 4c 89 ff e8 8d 85 6a 00 85 c0 74 0a <0f> 0b 4c 89 ff e8 8f 80 6a 00 65 48 8b 04 25 40 7f 01 00 83 a8 d8
[   97.413487] RSP: 0000:ffffb882493afd28 EFLAGS: 00010206
[   97.417520] RAX: 0000000000001000 RBX: ffffb882493afdf8 RCX: 0000000000001000
[   97.422295] RDX: 0000000000001000 RSI: 00007f1d20c00000 RDI: ffff976384d1f000
[   97.426914] RBP: 0000000000000000 R08: 0000000000000000 R09: 00000000000ca308
[   97.431746] R10: 0000000000000000 R11: ffffe0cd4c1347c0 R12: ffffe0cd4c1347c0
[   97.436371] R13: ffff9763b46ba190 R14: ffff9763a963d0c0 R15: ffff976384d1f000
[   97.441085] FS:  00007f1d203fe700(0000) GS:ffff9763b8a00000(0000) knlGS:0000000000000000
[   97.445500] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   97.448393] CR2: 00007f1d20c00000 CR3: 0000000333dfc000 CR4: 00000000000006f0
[   97.452346] Call Trace:
[   97.453681]  ? __switch_to_asm+0x34/0x70
[   97.455566]  ? __switch_to_asm+0x40/0x70
[   97.457418]  ? __switch_to_asm+0x34/0x70
[   97.459197]  ? __switch_to_asm+0x40/0x70
[   97.460971]  ? __switch_to_asm+0x34/0x70
[   97.462746]  ? __switch_to_asm+0x40/0x70
[   97.464561]  ? __switch_to_asm+0x34/0x70
[   97.466342]  ? __switch_to_asm+0x40/0x70
[   97.468141]  do_wp_page+0x8c/0x640
[   97.469818]  ? finish_task_switch+0x77/0x2a0
[   97.471631]  __handle_mm_fault+0xa06/0x1420
[   97.473517]  handle_mm_fault+0xae/0x1d0
[   97.475168]  __do_page_fault+0x27f/0x4e0
[   97.476947]  do_page_fault+0x30/0x110
[   97.478490]  async_page_fault+0x39/0x40
[   97.480275] RIP: 0033:0x400d68
[   97.481587] Code: 53 48 89 fb 48 83 ec 10 66 2e 0f 1f 84 00 00 00 00 00 0f b6 03 ba 04 00 00 00 be 00 00 20 00 48 89 df 89 44 24 0c 8b 44 24 0c <88> 03 e8 71 fc ff ff 85 c0 78 30 e8 b8 fc ff ff 89 c7 41 f7 ec 89
[   97.489326] RSP: 002b:00007f1d203fded0 EFLAGS: 00010202
[   97.491336] RAX: 0000000000000001 RBX: 00007f1d20c00000 RCX: 0000000000000000
[   97.494080] RDX: 0000000000000004 RSI: 0000000000200000 RDI: 00007f1d20c00000
[   97.497244] RBP: 0000000000000002 R08: 00007f1d2138d230 R09: 00007f1d2138d260
[   97.500028] R10: 00007f1d203fe9d0 R11: 0000000000000000 R12: 0000000051eb851f
[   97.502785] R13: 00007fff01d5607f R14: 0000000000000000 R15: 00007f1d203fdfc0
[   97.505546] ---[ end trace 18f1c94bd7c3d1e1 ]---

Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
---
 mm/memory.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 45442d9..e3a1dce 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2269,18 +2269,12 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
 
 	/*
 	 * This really shouldn't fail, because the page is there
-	 * in the page tables. But it might just be unreadable,
-	 * in which case we just give up and fill the result with
-	 * zeroes.
+	 * in the page tables. But it could happen during races,
+	 * or it might just be unreadable, in which cases we
+	 * just give up and fill the result with zeroes.
 	 */
-	if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
-		/*
-		 * Give a warn in case there can be some obscure
-		 * use-case
-		 */
-		WARN_ON_ONCE(1);
+	if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
 		clear_page(kaddr);
-	}
 
 	ret = true;
 
-- 
1.8.3.1
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