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aa42b3db | 1 | /* Test for file system hole support. |
2b778ceb | 2 | Copyright (C) 2018-2021 Free Software Foundation, Inc. |
aa42b3db FW |
3 | This file is part of the GNU C Library. |
4 | ||
5 | The GNU C Library is free software; you can redistribute it and/or | |
6 | modify it under the terms of the GNU Lesser General Public | |
7 | License as published by the Free Software Foundation; either | |
8 | version 2.1 of the License, or (at your option) any later version. | |
9 | ||
10 | The GNU C Library is distributed in the hope that it will be useful, | |
11 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
13 | Lesser General Public License for more details. | |
14 | ||
15 | You should have received a copy of the GNU Lesser General Public | |
16 | License along with the GNU C Library; if not, see | |
5a82c748 | 17 | <https://www.gnu.org/licenses/>. */ |
aa42b3db FW |
18 | |
19 | #include <stdbool.h> | |
20 | #include <support.h> | |
21 | #include <support/check.h> | |
22 | #include <sys/stat.h> | |
23 | #include <xunistd.h> | |
24 | ||
25 | int | |
26 | support_descriptor_supports_holes (int fd) | |
27 | { | |
28 | enum | |
29 | { | |
30 | /* Write offset for the enlarged file. This value is arbitrary | |
31 | and hopefully large enough to trigger the creation of holes. | |
32 | We cannot use the file system block size as a reference here | |
33 | because it is incorrect for network file systems. */ | |
34 | write_offset = 16 * 1024 * 1024, | |
35 | ||
36 | /* Our write may add this number of additional blocks (see | |
3f6e4fc4 ST |
37 | block_limit below): writing at offset 16M can require two data block |
38 | indirections, each of which can be as large as 8KB on ext2, thus 32 | |
39 | 512B sectors. */ | |
40 | block_headroom = 32, | |
aa42b3db FW |
41 | }; |
42 | ||
43 | struct stat64 st; | |
44 | xfstat (fd, &st); | |
45 | if (!S_ISREG (st.st_mode)) | |
46 | FAIL_EXIT1 ("descriptor %d does not refer to a regular file", fd); | |
47 | if (st.st_size != 0) | |
48 | FAIL_EXIT1 ("descriptor %d does not refer to an empty file", fd); | |
49 | if (st.st_blocks > block_headroom) | |
50 | FAIL_EXIT1 ("descriptor %d refers to a pre-allocated file (%lld blocks)", | |
51 | fd, (long long int) st.st_blocks); | |
52 | ||
53 | /* Write a single byte at the start of the file to compute the block | |
54 | usage for a single byte. */ | |
55 | xlseek (fd, 0, SEEK_SET); | |
56 | char b = '@'; | |
57 | xwrite (fd, &b, 1); | |
58 | /* Attempt to bypass delayed allocation. */ | |
59 | TEST_COMPARE (fsync (fd), 0); | |
60 | xfstat (fd, &st); | |
61 | ||
62 | /* This limit is arbitrary. The file system needs to store | |
63 | somewhere that data exists at the write offset, and this may | |
64 | moderately increase the number of blocks used by the file, in | |
65 | proportion to the initial block count, but not in proportion to | |
66 | the write offset. */ | |
67 | unsigned long long int block_limit = 2 * st.st_blocks + block_headroom; | |
68 | ||
69 | /* Write a single byte at 16 megabytes. */ | |
70 | xlseek (fd, write_offset, SEEK_SET); | |
71 | xwrite (fd, &b, 1); | |
72 | /* Attempt to bypass delayed allocation. */ | |
73 | TEST_COMPARE (fsync (fd), 0); | |
74 | xfstat (fd, &st); | |
75 | bool supports_holes = st.st_blocks <= block_limit; | |
76 | ||
77 | /* Also check that extending the file does not fill up holes. */ | |
78 | xftruncate (fd, 2 * write_offset); | |
79 | /* Attempt to bypass delayed allocation. */ | |
80 | TEST_COMPARE (fsync (fd), 0); | |
81 | xfstat (fd, &st); | |
82 | supports_holes = supports_holes && st.st_blocks <= block_limit; | |
83 | ||
84 | /* Return to a zero-length file. */ | |
85 | xftruncate (fd, 0); | |
86 | xlseek (fd, 0, SEEK_SET); | |
87 | ||
88 | return supports_holes; | |
89 | } |