From c74fbaefebe91ca60e3a8582613a3714c740df65 Mon Sep 17 00:00:00 2001 From: Jim Meyering Date: Fri, 17 Apr 2009 18:44:18 +0200 Subject: cp: work around linux kernel bug: short-read != EOF on /proc Remove the optimization that avoided up to 50% of cp's read syscalls. Do not assume that a short read on a regular file indicates EOF. When reading from a file in /proc on linux [at least 2.6.9 - 2.6.29] into a 4k-byte buffer or larger, a short read does not always indicate EOF. For example, "cp /proc/slabinfo /tmp" copies only 4068 of the total 7493 bytes. This optimization (25719a33154f0c62ea9881f0c79ae312dd4cec7a, Improve performance a bit by optimizing away; 2005-11-24) appears to have been worth less than a 2% speed-up (and usually much less), so the impact of removing it is negligible. * src/copy.c (copy_reg): Don't exit the loop early. * tests/cp/proc-short-read: New test, lightly based on a suggestion from Mike Frysinger, to exercise this fix. * tests/Makefile.am (TESTS): Add cp/proc-short-read. * NEWS (Improve robustness): Mention this change. --- src/copy.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) (limited to 'src') diff --git a/src/copy.c b/src/copy.c index 9b0e139c6..c45224c8a 100644 --- a/src/copy.c +++ b/src/copy.c @@ -700,9 +700,10 @@ copy_reg (char const *src_name, char const *dst_name, } last_write_made_hole = false; - /* A short read on a regular file means EOF. */ - if (n_read != buf_size && S_ISREG (src_open_sb.st_mode)) - break; + /* It is tempting to return early here upon a short read from a + regular file. That would save the final read syscall for each + file. Unfortunately that doesn't work for certain files in + /proc with linux kernels from at least 2.6.9 .. 2.6.29. */ } } -- cgit v1.2.3-54-g00ecf