add recommendation for using a large "len" argument

[rmacklem]

Summary

PR#252358 reported a serious performance problem w.r.t.
cp(1) when copying large non-sparse files.
This problem appears to have been caused by cp(1)
calling copy_file_range(2) with a small "len" argument.

This patch adds a recommendation to use a large "len"
value where possible, for performance reasons.

[asomers]

lib/libc/sys/copy_file_range.2

136

I think this belongs in the DESCRIPTION section, not RETURN VALUES.

139

Why SSIZE_MAX? Since len is defined as a size_t, shouldn't the limit be SIZE_MAX ?

141

I think this qualifies as TMI. I might phrase it as "For best performance, call copy_file_range with the largest len value possible. It is interruptible on most file systems, so there is no penalty for using very large len values, even SSIZE_MAX."

[rmacklem]

Update patch as recommended by asomers@.

[rmacklem]

Handled inline comments.

lib/libc/sys/copy_file_range.2

139

Because it returns ssize_t.
I didn't think that needed to be stated here,
but if you think it should be, I can add a
sentence.

[asomers]

lib/libc/sys/copy_file_range.2

139

Ahh, that makes sense.