>>> Hugh Dickins <hu...@google.com> schrieb am 04.08.2013 um 00:37 in Nachricht
<alpine.LNX.2.00.1308031516010.11134@eggly.anvils>:
> On Thu, 1 Aug 2013, Ulrich Windl wrote:
>> Hi folks!
>>
>> I think I'd let you know (maybe I'm wrong, and the kernel is right):
>>
>> I write a C-program that maps a file into an private writable map. Then I
> modify the area a bit and use one write to write that area back to a file.
>>
>> This worked fine in SLES11 kernel 3.0.74-0.6.10. However with kernel
> 3.0.80-0.7 the write() fails with EFAULT if the output file is the same as
> the input file.
>
> I wonder if you actually did exactly the same on both kernels.
Hi!
thanks for replying! Actually id did the sam a few thousand times (with
different files and different lengths) in the previous kernel, weher it never
failed, just as with the newer kernel where it always fails (it seems).
>
>>
>> The strace is amazingly short (I removed the unrelated calls):
>
> Providing that was very helpful.
>
>> open("xxx", O_RDONLY) = 3
>> fstat(3, {st_mode=S_IFREG|0644, st_size=4416, ...}) = 0
>> mmap(NULL, 4416, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x7f85ac045000
>> close(3) = 0
>> open("xxx", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
>
> The crucial point is the above O_TRUNC when you now open the file for
> writing: that truncates the file to 0-length, which unmaps any pages
> mapped from it into userspace. Even the privately modified COW pages:
Well, but the mapping is PRIVATE, so I guessed once mapped, changes to the map
won't affect the file, just as changes to the file won't affect the map.
Specifically when re-opening the file for writing with O_TRUNC I did not expect
the map to become invalid. Also note that the unmap still returns no error.
My manual page vaguely says: "It is unspecified whether changes made to the
file after the mmap() call are visible in the mapped region."
> that often seems surprising, but it is how mmap versus truncate is
> specified to work.
>
>> write(3, 0x7f85ac045000, 4414) = -1 EFAULT (Bad address)
>
> If your program now touched a part of the mapping, it would get
> SIGBUS, there being no pages of underlying object to page in from.
> But since you're accessing the area from within a system call,
> that simply fails with EFAULT.
OK, if things are like this, the older kernel must have been faulty.
>
>> close(3) = 0
>> munmap(0x7f85ac045000, 4414) = 0
>>
>> I want to have your attention if this should work, and you get my attention
> if this should not work.
>
> It should not work.
>
>> Note that the input file is closed before it's opened for write again. As
> the output file is typically shorter than the input, I didn't want to use a
> non-private mapping and a truncate, just in case you wonder...
>
> (I didn't understand your logic there.)
The alternative to write() a part of the PRIVATE area would be to work with a
non-PRIVATE area that is truncated after flushing the changes. In principle the
same blocks could be written multiple times (when you move data from later
parts to earlier parts (i.e.: from the far end closer to the beginning)), so I
thought a PRIVATE mapping plus one write() would avoid that. I had the coice of
truncate while opening, or to truncate the extra data after write(). I chose
the first alternative.
Maybe I'll re-design...
Thanks,
Ulrich
>
> Hugh
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