Unify fs::copy and io::copy on Linux

[SUPERCILEX]

Currently, fs::copy first tries a regular file copy (via copy_file_range) and then falls back to userspace read/write copying. We should use io::copy instead as it tries copy_file_range, sendfile, and splice before falling back to userspace copying. This was discovered here: SUPERCILEX/fuc#40

Perf impact: fs::copy will now have two additional statx calls to decide which syscall to use. I wonder if we should get rid of the statx calls and only continue down the next fallback when the relevant syscalls say the FD isn't supported.

[rustbot]

r? @thomcc

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[thomcc]

@bors try @rust-timer queue

[thomcc]

It would be nice to dedupe the stat calls, but I assume that's too much of a pain?

[bors]

⌛ Trying commit 2b4334e with merge d179dd5...

[SUPERCILEX]

Sorry, what do you mean by "dedupe the stat calls?" There aren't unnecessary calls—we use them to determine which syscall to use. I'm suggesting that instead of doing stat -> stat -> relevant_syscall, we can do failed_syscall -> failed_syscall -> failed_syscall -> use space copy in the worst case and go straight to copy_file_range in the fast path. So worst case it's an extra syscall but best case using any of the copy acceleration syscalls is no more than paying the stat calls.

[thomcc]

Ah, I had misunderstood. We can leave something like that for a future pr.

[SUPERCILEX]

Sounds good! I guess do we know what proportion of calls of fs/io copy are between regular files vs pipes vs sockets etc? No idea how we'd get this data lol.

[bors]

☀️ Try build successful - checks-actions
Build commit: d179dd5 (d179dd52797cf4ff504a7914bf84a41fd5603089)

[rust-timer]

Finished benchmarking commit (d179dd5): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results (secondary 3.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 767.128s -> 766.815s (-0.04%)
Artifact size: 330.26 MiB -> 330.24 MiB (-0.00%)

[thomcc]

@bors r+

[bors]

📌 Commit 2b4334e has been approved by thomcc

It is now in the queue for this repository.

[SUPERCILEX]

Ok I took a look at the io::copy code and #108283 means we can't skip the stat calls for sendfile, so avoiding the stat calls for copy_file_range probably isn't worth it.

Though also I lied and there are duplicate stat calls! Will fix.

[SUPERCILEX]

Ok, fixed. Old:

openat(AT_FDCWD, "LICENSE", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=11358, ...}) = 0
openat(AT_FDCWD, "/tmp/LICENSE", O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0100664) = 4
statx(4, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=0, ...}) = 0
fchmod(4, 0100664)                      = 0
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=11358, ...}) = 0
statx(4, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=0, ...}) = 0
copy_file_range(3, NULL, 4, NULL, 1073741824, 0) = -1 EXDEV (Invalid cross-device link)
sendfile(4, 3, NULL, 2147479552)        = 11358
sendfile(4, 3, NULL, 2147479552)        = 0
close(4)                                = 0
close(3)                                = 0

New:

openat(AT_FDCWD, "LICENSE", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=11358, ...}) = 0
openat(AT_FDCWD, "/tmp/LICENSE", O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0100664) = 4
statx(4, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=0, ...}) = 0
fchmod(4, 0100664)                      = 0
copy_file_range(3, NULL, 4, NULL, 1073741824, 0) = -1 EXDEV (Invalid cross-device link)
sendfile(4, 3, NULL, 2147479552)        = 11358
sendfile(4, 3, NULL, 2147479552)        = 0
close(4)                                = 0
close(3)                                = 0

[rustbot]

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[the8472]

The kernel_copy module is only defined for linux/android. So this would fail on some other unixes.