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xfs_io: fix random pread/pwrite to honor offset

PATCH In review Eric Sandeen 3 participants 4 messages Apr 9, 2014 – Apr 11, 2014

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Eric Sandeen

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#0 lore raw

PATCH 1/1 xfs_io: fix random pread/pwrite to honor offset

xfs_io's pread & pwrite claim to support a random IO mode where it will do random IOs between offset & offset+len.

However, offset was ignored, and we did the IOs between 0 and len instead.

Clang caught this by pointing out that the calculated/normalized "offset" variable was never read.

(NB: If the range is larger than RAND_MAX, these functions don't work, but that's always been true, so I'll leave it for another day...)

Signed-off-by Eric Sandeen <[email protected]>

---

I haven't tested this w/ xfstests but I don't see anyone who calls pread/pwrite with "-R" (and it's not documented in the manpage!)

io/pread.c +1 -1
index a42baed..465c22b 100644
@@ -246,7 +246,7 @@ read_random(
 
 	*total = 0;
 	while (count > 0) {
-		off = ((random() % range) / buffersize) * buffersize;
+		off = ((offset + (random() % range)) / buffersize) * buffersize;
 		bytes = do_pread(fd, off, buffersize, buffersize);
 		if (bytes == 0)
 			break;
io/pwrite.c +1 -1
index a2f0a81..f8a0b1e 100644
@@ -129,7 +129,7 @@ write_random(
 
 	*total = 0;
 	while (count > 0) {
-		off = ((random() % range) / buffersize) * buffersize;
+		off = ((offset + (random() % range)) / buffersize) * buffersize;
 		bytes = do_pwrite(file->fd, off, buffersize, buffersize);
 		if (bytes == 0)
 			break;

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#1 lore raw

io/pread.c read_random(

Quoted 24 lines · Eric Sandeen

xfs_io's pread & pwrite claim to support a random IO mode where it will do random IOs between offset & offset+len.

However, offset was ignored, and we did the IOs between 0 and len instead.

Clang caught this by pointing out that the calculated/normalized "offset" variable was never read.

(NB: If the range is larger than RAND_MAX, these functions don't work, but that's always been true, so I'll leave it for another day...)

Signed-off-by Eric Sandeen<[email protected]>

---

I haven't tested this w/ xfstests but I don't see anyone who calls pread/pwrite with "-R" (and it's not documented in the manpage!)

io/pread.c +0 -0
index a42baed..465c22b 100644
@@ -246,7 +246,7 @@ read_random(

offset can be the end of file offset. Do we care that the read (/write) could start or end past the end of file?

io/pread.c read_random( · line 252

Quoted 8 lines
io/pread.c +1 -1
  	*total = 0;
  	while (count > 0) {
-		off = ((random() % range) / buffersize) * buffersize;
+		off = ((offset + (random() % range)) / buffersize) * buffersize;
  		bytes = do_pread(fd, off, buffersize, buffersize);
  		if (bytes == 0)
  			break;

Looks like this was introduced in:
commit 8fb2237e65555ff540e8b6108ffccfffefe239ac

Author: Nathan Scott <[email protected]>
Date:   Fri Nov 11 14:25:18 2005 +0000

Provide further debugging options and tweaks for analysing the read/write paths. Merge of master-melb:xfs-cmds:24372a by kenmcd.

---

If it was broken for 8.5 years, I think it could be removed.

--Mark.

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author
#2 lore raw
Quoted 20 lines · Mark Tinguely
Quoted 19 lines · Eric Sandeen

xfs_io's pread & pwrite claim to support a random IO mode where it will do random IOs between offset & offset+len.

However, offset was ignored, and we did the IOs between 0 and len instead.

Clang caught this by pointing out that the calculated/normalized "offset" variable was never read.

(NB: If the range is larger than RAND_MAX, these functions don't work, but that's always been true, so I'll leave it for another day...)

Signed-off-by Eric Sandeen<[email protected]>

---

I haven't tested this w/ xfstests but I don't see anyone who calls pread/pwrite with "-R" (and it's not documented in the manpage!)

Quoted 8 lines

io/pread.c read_random(

Quoted 5 lines
io/pread.c +0 -0
index a42baed..465c22b 100644
@@ -246,7 +246,7 @@ read_random(

offset can be the end of file offset. Do we care that the read (/write) could start or end past the end of file?

It now behaves the same as a normal forward read does if you tell it to go past EOF:

xfs_io> truncate 1g
xfs_io> pread 1g 1m
read 0/1048576 bytes at offset 1073741824
0.000000 bytes, 0 ops; 0.0000 sec (0.000000 bytes/sec and 0.0000 ops/sec) xfs_io> pread -R 1g 1m
read 0/1048576 bytes at offset 1073741824
0.000000 bytes, 0 ops; 0.0000 sec (0.000000 bytes/sec and 0.0000 ops/sec)

so I think it's ok?

Quoted 14 lines

io/pread.c read_random( · line 252

Quoted 8 lines
io/pread.c +1 -1
      *total = 0;
      while (count > 0) {
-        off = ((random() % range) / buffersize) * buffersize;
+        off = ((offset + (random() % range)) / buffersize) * buffersize;
          bytes = do_pread(fd, off, buffersize, buffersize);
          if (bytes == 0)
              break;

Looks like this was introduced in:
commit 8fb2237e65555ff540e8b6108ffccfffefe239ac

Author: Nathan Scott <[email protected]>
Date:   Fri Nov 11 14:25:18 2005 +0000
Quoted 6 lines

Provide further debugging options and tweaks for analysing the read/write paths. Merge of master-melb:xfs-cmds:24372a by kenmcd.

---

If it was broken for 8.5 years, I think it could be removed.

Eh, could, or we could fix it. :) I suppose this means it needs a test... :/

-Eric

Quoted 2 lines

--Mark.

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#3 lore raw
Quoted 17 lines · Eric Sandeen
Quoted 16 lines · Mark Tinguely
Quoted 15 lines · Eric Sandeen

xfs_io's pread & pwrite claim to support a random IO mode where it will do random IOs between offset & offset+len.

However, offset was ignored, and we did the IOs between 0 and len instead.

Clang caught this by pointing out that the calculated/normalized "offset" variable was never read.

(NB: If the range is larger than RAND_MAX, these functions don't work, but that's always been true, so I'll leave it for another day...)

Signed-off-by Eric Sandeen<[email protected]>

---

....

Quoted 6 lines
Quoted 6 lines

Looks like this was introduced in:
commit 8fb2237e65555ff540e8b6108ffccfffefe239ac

Author: Nathan Scott <[email protected]>
Date:   Fri Nov 11 14:25:18 2005 +0000
Quoted 8 lines
Quoted 6 lines

Provide further debugging options and tweaks for analysing the read/write paths. Merge of master-melb:xfs-cmds:24372a by kenmcd.

---

If it was broken for 8.5 years, I think it could be removed.

Eh, could, or we could fix it. :) I suppose this means it needs a test... :/

And document it ;)

Cheers,

Dave.

Signature · 7 lines
Dave Chinner
[email protected]

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