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From: Arnaldo Carvalho de Melo on 3 Aug 2010 10:40
Em Tue, Aug 03, 2010 at 12:48:35PM +0100, Dave Martin escreveu:
> Fix buggy-looking code which unnecessarily adjusts the file offset
> fields read from /proc/*/maps.
>
> This may have gone unnoticed since the offset is usually 0 (and the
> logic in util/symbol.c may work incorrectly for other offset values).
>
> I make assumptions about the intended design here. The cover note
> accompanying this patch contains a more detailed explanation.
>
> Signed-off-by: Dave Martin <dave.martin(a)linaro.org>

Doing some investigation here...

4af8b35d (Anton Blanchard 2010-04-03 164) pbf += 3;
4af8b35d (Anton Blanchard 2010-04-03 165) n = hex2u64(pbf, &vm_pgoff);
4af8b35d (Anton Blanchard 2010-04-03 166) /* pgoff is in bytes, not pages */
4af8b35d (Anton Blanchard 2010-04-03 167) if (n >= 0)
4af8b35d (Anton Blanchard 2010-04-03 168) ev.mmap.pgoff = vm_pgoff << getpagesize();
4af8b35d (Anton Blanchard 2010-04-03 169) else
4af8b35d (Anton Blanchard 2010-04-03 170) ev.mmap.pgoff = 0;

commit 4af8b35db6634dd1e0d616de689582b6c93550af
Author: Anton Blanchard <anton(a)samba.org>
Date: Sat Apr 3 22:53:31 2010 +1100

perf symbols: Fill in pgoff in mmap synthesized events

When we synthesize mmap events we need to fill in the pgoff field.

I wasn't able to test this completely since I couldn't find an
executable region with a non 0 offset. We will see it when we start
doing data profiling.

------------------------

Yeah, not reassuring comment, looking at fs/proc/task_mmu.c we see:

static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
{
struct mm_struct *mm = vma->vm_mm;
struct file *file = vma->vm_file;
int flags = vma->vm_flags;
unsigned long ino = 0;
unsigned long long pgoff = 0;
dev_t dev = 0;
int len;

if (file) {
struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
dev = inode->i_sb->s_dev;
ino = inode->i_ino;
pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
}

seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
vma->vm_start,
vma->vm_end,
flags & VM_READ ? 'r' : '-',
flags & VM_WRITE ? 'w' : '-',
flags & VM_EXEC ? 'x' : '-',
flags & VM_MAYSHARE ? 's' : 'p',
pgoff,
MAJOR(dev), MINOR(dev), ino, &len);

----------------------------------------------------------------------------

So yes, we're double shifting that, your patch is correct, applying it.

> ---
> tools/perf/util/event.c | 8 +-------
> 1 files changed, 1 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> index 6b0db55..db8a1d4 100644
> --- a/tools/perf/util/event.c
> +++ b/tools/perf/util/event.c
> @@ -151,7 +151,6 @@ static int event__synthesize_mmap_events(pid_t pid, pid_t tgid,
> continue;
> pbf += n + 3;
> if (*pbf == 'x') { /* vm_exec */
> - u64 vm_pgoff;
> char *execname = strchr(bf, '/');
>
> /* Catch VDSO */
> @@ -162,12 +161,7 @@ static int event__synthesize_mmap_events(pid_t pid, pid_t tgid,
> continue;
>
> pbf += 3;
> - n = hex2u64(pbf, &vm_pgoff);
> - /* pgoff is in bytes, not pages */
> - if (n >= 0)
> - ev.mmap.pgoff = vm_pgoff << getpagesize();
> - else
> - ev.mmap.pgoff = 0;
> + n = hex2u64(pbf, &ev.mmap.pgoff);
>
> size = strlen(execname);
> execname[size - 1] = '\0'; /* Remove \n */
> --
> 1.7.0.4
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