Re: [fuse-devel] [fuse] Getting visibility into reads from page cache

[Nikolaus Rath <Nikolaus@rath.org>]

On May 08 2020, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>
>>   sudo bpftrace -e 'kretprobe:fuse_file_read_iter { printf ("fuse
>> read: %d\n", retval); }'
>
>
> - I believe that (struct kiocb*)arg0)->ki_pos will give me the offset
>   within the file, but where can I see how much data is being read?

Looking at the code in fuse_file_read_iter, it seems the length is in
((struct iov_iter*)arg1)->count, but I do not really understand why.

The definiton of this parameter is:

struct iov_iter {
	int type;
	const struct iovec *iov;
	unsigned long nr_segs;
	size_t iov_offset;
	size_t count;
};

..so I would think that *count* is the number of `iovec` elements hiding
behind the `iov` pointer, not some total number of bytes.

Furthermore, there is a function iov_length() that is documented to
return the "total number of bytes covered by an iovec" and doesn't look
at `count` at all.

Can someone elucidate why the number of bytes to be read from the file
is in iov_iter.count?


Best,
-Nikolaus

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