Re: WARN_ON_ONCE(1) in iomap_dio_actor()

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jun 19, 2020 at 05:17:50PM -0400, Qian Cai wrote:
> Running a syscall fuzzer by a normal user could trigger this,
> 
> [55649.329999][T515839] WARNING: CPU: 6 PID: 515839 at fs/iomap/direct-io.c:391 iomap_dio_actor+0x29c/0x420
...
> 371 static loff_t
> 372 iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> 373                 void *data, struct iomap *iomap, struct iomap *srcmap)
> 374 {
> 375         struct iomap_dio *dio = data;
> 376
> 377         switch (iomap->type) {
> 378         case IOMAP_HOLE:
> 379                 if (WARN_ON_ONCE(dio->flags & IOMAP_DIO_WRITE))
> 380                         return -EIO;
> 381                 return iomap_dio_hole_actor(length, dio);
> 382         case IOMAP_UNWRITTEN:
> 383                 if (!(dio->flags & IOMAP_DIO_WRITE))
> 384                         return iomap_dio_hole_actor(length, dio);
> 385                 return iomap_dio_bio_actor(inode, pos, length, dio, iomap);
> 386         case IOMAP_MAPPED:
> 387                 return iomap_dio_bio_actor(inode, pos, length, dio, iomap);
> 388         case IOMAP_INLINE:
> 389                 return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
> 390         default:
> 391                 WARN_ON_ONCE(1);
> 392                 return -EIO;
> 393         }
> 394 }
> 
> Could that be iomap->type == IOMAP_DELALLOC ? Looking throught the logs,
> it contains a few pread64() calls until this happens,

It _shouldn't_ be able to happen.  XFS writes back ranges which exist
in the page cache upon seeing an O_DIRECT I/O.  So it's not supposed to
be possible for there to be an extent which is waiting for the contents
of the page cache to be written back.

> [child21:124180] [17] pread64(fd=353, buf=0x0, count=0x59b5, pos=0xe0e0e0e) = -1 (Illegal seek)
> [child21:124180] [339] pread64(fd=339, buf=0xffffbcc40000, count=0xbd71, pos=0xff26) = -1 (Illegal seek)
> [child21:124627] [136] pread64(fd=69, buf=0xffffbd290000, count=0xee42, pos=2) = -1 (Illegal seek)
> [child21:124627] [196] pread64(fd=83, buf=0x1, count=0x62f8, pos=0x15390000) = -1 (Illegal seek)
> [child21:125127] [154] pread64(fd=345, buf=0xffffbcc40000, count=9332, pos=0xffbd) = 9332
> [child21:125169] [188] pread64(fd=69, buf=0xffffbce90000, count=0x4d47, pos=0) = -1 (Illegal seek)
> [child21:125169] [227] pread64(fd=345, buf=0x1, count=0xe469, pos=1046) = -1 (Bad address)
> [child21:125569] [354] pread64(fd=87, buf=0xffffbcc50000, count=0x4294, pos=0x16161616) = -1 (Illegal seek)
> [child21:125569] [655] pread64(fd=341, buf=0xffffbcc70000, count=2210, pos=0xffff) = -1 (Illegal seek)
> [child21:125569] [826] pread64(fd=343, buf=0x8, count=0xeb22, pos=0xc090c202e598b) = 0
> [child21:126233] [261] pread64(fd=338, buf=0xffffbcc40000, count=0xe8fe, pos=105) = -1 (Illegal seek)
> [child21:126233] [275] pread64(fd=190, buf=0x8, count=0x9c24, pos=116) = -1 (Is a directory)
> [child21:126882] [32] pread64(fd=86, buf=0xffffbcc40000, count=0x7fc2, pos=2) = -1 (Illegal seek)
> [child21:127448] [14] pread64(fd=214, buf=0x4, count=11371, pos=0x9b26) = 0
> [child21:127489] [70] pread64(fd=339, buf=0xffffbcc70000, count=0xb07a, pos=8192) = -1 (Illegal seek)
> [child21:127489] [80] pread64(fd=339, buf=0x0, count=6527, pos=205) = -1 (Illegal seek)
> [child21:127489] [245] pread64(fd=69, buf=0x8, count=0xbba2, pos=47) = -1 (Illegal seek)
> [child21:128098] [334] pread64(fd=353, buf=0xffffbcc90000, count=0x4540, pos=168) = -1 (Illegal seek)
> [child21:129079] [157] pread64(fd=422, buf=0x0, count=0x80df, pos=0xdfef6378b650aa) = 0
> [child21:134700] [275] pread64(fd=397, buf=0xffffbcc50000, count=0xdee6, pos=0x887b1e74a2) = -1 (Illegal seek)
> [child21:135042] [7] pread64(fd=80, buf=0x8, count=0xc494, pos=216) = -1 (Illegal seek)
> [child21:135056] [188] pread64(fd=430, buf=0xffffbd090000, count=0xbe66, pos=0x3a3a3a3a) = -1 (Illegal seek)
> [child21:135442] [143] pread64(fd=226, buf=0xffffbd390000, count=11558, pos=0x1000002d) = 0
> [child21:135513] [275] pread64(fd=69, buf=0x4, count=4659, pos=0x486005206c2986) = -1 (Illegal seek)
> [child21:135513] [335] pread64(fd=339, buf=0xffffbd090000, count=0x90fd, pos=253) = -1 (Illegal seek)
> [child21:135513] [392] pread64(fd=76, buf=0xffffbcc40000, count=0xf324, pos=0x5d5d5d5d) = -1 (Illegal seek)
> [child21:135665] [5] pread64(fd=431, buf=0xffffbcc70000, count=10545, pos=16384) = -1 (Illegal seek)
> [child21:135665] [293] pread64(fd=349, buf=0x4, count=0xd2ad, pos=0x2000000) = -1 (Illegal seek)
> [child21:135790] [99] pread64(fd=76, buf=0x8, count=0x70d7, pos=0x21000440) = -1 (Illegal seek)
> [child21:135790] [149] pread64(fd=70, buf=0xffffbd5b0000, count=0x53f3, pos=255) = -1 (Illegal seek)
> [child21:135790] [301] pread64(fd=348, buf=0x4, count=5713, pos=0x6c00401a) = -1 (Illegal seek)
> [child21:136162] [570] pread64(fd=435, buf=0x1, count=11182, pos=248) = -1 (Illegal seek)
> [child21:136162] [584] pread64(fd=78, buf=0xffffbcc40000, count=0xa401, pos=8192) = -1 (Illegal seek)
> [child21:138090] [167] pread64(fd=339, buf=0x4, count=0x6aba, pos=256) = -1 (Illegal seek)
> [child21:138090] [203] pread64(fd=348, buf=0xffffbcc90000, count=0x8625, pos=128) = -1 (Illegal seek)
> [child21:138551] [174] pread64(fd=426, buf=0x0, count=0xd582, pos=0xd7e8674d0a86) = 0
> [child21:138551] [179] pread64(fd=426, buf=0xffffbce90000, count=0x415a, pos=0x536e873600750b2d) = 0
> [child21:138988] [306] pread64(fd=436, buf=0x8, count=0x62e6, pos=0x445c403204924c1) = -1 (Illegal seek)
> [child21:138988] [353] pread64(fd=427, buf=0x4, count=0x993b, pos=176) = 0