Re: [PATCH v3 04/13] ltp/fsx.c: Add atomic writes support to fsx

Darrick J. Wong
2025-07-17T16:17:48Z

On Sat, Jul 12, 2025 at 07:42:46PM +0530, Ojaswin Mujoo wrote:
> Implement atomic write support to help fuzz atomic writes
> with fsx.
>
> Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> ---
> ltp/fsx.c | 109 +++++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 104 insertions(+), 5 deletions(-)
>
> diff --git a/ltp/fsx.c b/ltp/fsx.c
> index 163b9453..ea39ca29 100644
> --- a/ltp/fsx.c
> +++ b/ltp/fsx.c
> @@ -40,6 +40,7 @@
> #include <liburing.h>
> #endif
> #include <sys/syscall.h>
> +#include "statx.h"
>
> #ifndef MAP_FILE
> # define MAP_FILE 0
> @@ -49,6 +50,10 @@
> #define RWF_DONTCACHE 0x80
> #endif
>
> +#ifndef RWF_ATOMIC
> +#define RWF_ATOMIC 0x40
> +#endif
> +
> #define NUMPRINTCOLUMNS 32 /* # columns of data to print on each line */
>
> /* Operation flags (bitmask) */
> @@ -110,6 +115,7 @@ enum {
> OP_READ_DONTCACHE,
> OP_WRITE,
> OP_WRITE_DONTCACHE,
> + OP_WRITE_ATOMIC,
> OP_MAPREAD,
> OP_MAPWRITE,
> OP_MAX_LITE,
> @@ -200,6 +206,11 @@ int uring = 0;
> int mark_nr = 0;
> int dontcache_io = 1;
> int hugepages = 0; /* -h flag */
> +int do_atomic_writes = 1; /* -a flag disables */
> +
> +/* User for atomic writes */
> +int awu_min = 0;
> +int awu_max = 0;
>
> /* Stores info needed to periodically collapse hugepages */
> struct hugepages_collapse_info {
> @@ -288,6 +299,7 @@ static const char *op_names[] = {
> [OP_READ_DONTCACHE] = "read_dontcache",
> [OP_WRITE] = "write",
> [OP_WRITE_DONTCACHE] = "write_dontcache",
> + [OP_WRITE_ATOMIC] = "write_atomic",
> [OP_MAPREAD] = "mapread",
> [OP_MAPWRITE] = "mapwrite",
> [OP_TRUNCATE] = "truncate",
> @@ -422,6 +434,7 @@ logdump(void)
> prt("\t***RRRR***");
> break;
> case OP_WRITE_DONTCACHE:
> + case OP_WRITE_ATOMIC:
> case OP_WRITE:
> prt("WRITE 0x%x thru 0x%x\t(0x%x bytes)",
> lp->args[0], lp->args[0] + lp->args[1] - 1,
> @@ -1073,6 +1086,25 @@ update_file_size(unsigned offset, unsigned size)
> file_size = offset + size;
> }
>
> +static int is_power_of_2(unsigned n) {
> + return ((n & (n - 1)) == 0);
> +}
> +
> +/*
> + * Round down n to nearest power of 2.
> + * If n is already a power of 2, return n;
> + */
> +static int rounddown_pow_of_2(int n) {
> + int i = 0;
> +
> + if (is_power_of_2(n))
> + return n;
> +
> + for (; (1 << i) < n; i++);
> +
> + return 1 << (i - 1);
> +}
> +
> void
> dowrite(unsigned offset, unsigned size, int flags)
> {
> @@ -1081,6 +1113,27 @@ dowrite(unsigned offset, unsigned size, int flags)
> offset -= offset % writebdy;
> if (o_direct)
> size -= size % writebdy;
> + if (flags & RWF_ATOMIC) {
> + /* atomic write len must be inbetween awu_min and awu_max */
> + if (size < awu_min)
> + size = awu_min;
> + if (size > awu_max)
> + size = awu_max;
> +
> + /* atomic writes need power-of-2 sizes */
> + size = rounddown_pow_of_2(size);
> +
> + /* atomic writes need naturally aligned offsets */
> + offset -= offset % size;

I don't think you should be modifying offset/size here. Normally for
fsx we do all the rounding of the file range in the switch statement
after the "calculate appropriate op to run" comment statement.

--D

> +
> + /* Skip the write if we are crossing max filesize */
> + if ((offset + size) > maxfilelen) {
> + if (!quiet && testcalls > simulatedopcount)
> + prt("skipping atomic write past maxfilelen\n");
> + log4(OP_WRITE_ATOMIC, offset, size, FL_SKIPPED);
> + return;
> + }
> + }
> if (size == 0) {
> if (!quiet && testcalls > simulatedopcount && !o_direct)
> prt("skipping zero size write\n");
> @@ -1088,7 +1141,10 @@ dowrite(unsigned offset, unsigned size, int flags)
> return;
> }
>
> - log4(OP_WRITE, offset, size, FL_NONE);
> + if (flags & RWF_ATOMIC)
> + log4(OP_WRITE_ATOMIC, offset, size, FL_NONE);
> + else
> + log4(OP_WRITE, offset, size, FL_NONE);
>
> gendata(original_buf, good_buf, offset, size);
> if (offset + size > file_size) {
> @@ -1108,8 +1164,9 @@ dowrite(unsigned offset, unsigned size, int flags)
> (monitorstart == -1 ||
> (offset + size > monitorstart &&
> (monitorend == -1 || offset <= monitorend))))))
> - prt("%lld write\t0x%x thru\t0x%x\t(0x%x bytes)\tdontcache=%d\n", testcalls,
> - offset, offset + size - 1, size, (flags & RWF_DONTCACHE) != 0);
> + prt("%lld write\t0x%x thru\t0x%x\t(0x%x bytes)\tdontcache=%d atomic_wr=%d\n", testcalls,
> + offset, offset + size - 1, size, (flags & RWF_DONTCACHE) != 0,
> + (flags & RWF_ATOMIC) != 0);
> iret = fsxwrite(fd, good_buf + offset, size, offset, flags);
> if (iret != size) {
> if (iret == -1)
> @@ -1785,6 +1842,30 @@ do_dedupe_range(unsigned offset, unsigned length, unsigned dest)
> }
> #endif
>
> +int test_atomic_writes(void) {
> + int ret;
> + struct statx stx;
> +
> + ret = xfstests_statx(AT_FDCWD, fname, 0, STATX_WRITE_ATOMIC, &stx);
> + if (ret < 0) {
> + fprintf(stderr, "main: Statx failed with %d."
> + " Failed to determine atomic write limits, "
> + " disabling!\n", ret);
> + return 0;
> + }
> +
> + if (stx.stx_attributes & STATX_ATTR_WRITE_ATOMIC &&
> + stx.stx_atomic_write_unit_min > 0) {
> + awu_min = stx.stx_atomic_write_unit_min;
> + awu_max = stx.stx_atomic_write_unit_max;
> + return 1;
> + }
> +
> + fprintf(stderr, "main: IO Stack does not support "
> + "atomic writes, disabling!\n");
> + return 0;
> +}
> +
> #ifdef HAVE_COPY_FILE_RANGE
> int
> test_copy_range(void)
> @@ -2356,6 +2437,12 @@ have_op:
> goto out;
> }
> break;
> + case OP_WRITE_ATOMIC:
> + if (!do_atomic_writes) {
> + log4(OP_WRITE_ATOMIC, offset, size, FL_SKIPPED);
> + goto out;
> + }
> + break;
> }
>
> switch (op) {
> @@ -2385,6 +2472,11 @@ have_op:
> dowrite(offset, size, 0);
> break;
>
> + case OP_WRITE_ATOMIC:
> + TRIM_OFF_LEN(offset, size, maxfilelen);
> + dowrite(offset, size, RWF_ATOMIC);
> + break;
> +
> case OP_MAPREAD:
> TRIM_OFF_LEN(offset, size, file_size);
> domapread(offset, size);
> @@ -2511,13 +2603,14 @@ void
> usage(void)
> {
> fprintf(stdout, "usage: %s",
> - "fsx [-dfhknqxyzBEFHIJKLORWXZ0]\n\
> + "fsx [-adfhknqxyzBEFHIJKLORWXZ0]\n\
> [-b opnum] [-c Prob] [-g filldata] [-i logdev] [-j logid]\n\
> [-l flen] [-m start:end] [-o oplen] [-p progressinterval]\n\
> [-r readbdy] [-s style] [-t truncbdy] [-w writebdy]\n\
> [-A|-U] [-D startingop] [-N numops] [-P dirpath] [-S seed]\n\
> [--replay-ops=opsfile] [--record-ops[=opsfile]] [--duration=seconds]\n\
> ... fname\n\
> + -a: disable atomic writes\n\
> -b opnum: beginning operation number (default 1)\n\
> -c P: 1 in P chance of file close+open at each op (default infinity)\n\
> -d: debug output for all operations\n\
> @@ -3059,9 +3152,13 @@ main(int argc, char **argv)
> setvbuf(stdout, (char *)0, _IOLBF, 0); /* line buffered stdout */
>
> while ((ch = getopt_long(argc, argv,
> - "0b:c:de:fg:hi:j:kl:m:no:p:qr:s:t:uw:xyABD:EFJKHzCILN:OP:RS:UWXZ",
> + "0ab:c:de:fg:hi:j:kl:m:no:p:qr:s:t:uw:xyABD:EFJKHzCILN:OP:RS:UWXZ",
> longopts, NULL)) != EOF)
> switch (ch) {
> + case 'a':
> + prt("main(): Atomic writes disabled\n");
> + do_atomic_writes = 0;
> + break;
> case 'b':
> simulatedopcount = getnum(optarg, &endp);
> if (!quiet)
> @@ -3475,6 +3572,8 @@ main(int argc, char **argv)
> exchange_range_calls = test_exchange_range();
> if (dontcache_io)
> dontcache_io = test_dontcache_io();
> + if (do_atomic_writes)
> + do_atomic_writes = test_atomic_writes();
>
> while (keep_running())
> if (!test())
> --
> 2.49.0
>
>