linux-kernel - RE: [PATCH] perf map: fix overlapped map handling

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Date:   Fri, 20 Sep 2019 21:46:15 +0000
From:   Steve MacLean <Steve.MacLean@...rosoft.com>
To:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Changbin Du <changbin.du@...el.com>,
        Eric Saint-Etienne <eric.saint.etienne@...cle.com>,
        John Keeping <john@...anate.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Leo Yan <leo.yan@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Robbins <brianrob@...rosoft.com>,
        Tom McDonald <Thomas.McDonald@...rosoft.com>,
        John Salem <josalem@...rosoft.com>
Subject: RE: [PATCH] perf map: fix overlapped map handling

>>  			after->start = map->end;
>> +			after->pgoff = pos->map_ip(pos, map->end);
>
> So is this equivalent to what __split_vma() does in the kernel, i.e.:
>
>        if (new_below)
>                new->vm_end = addr;
>        else {
>                new->vm_start = addr;
>                new->vm_pgoff += ((addr - vma->vm_start) >> PAGE_SHIFT);
>        }
>
> where new->vm_pgoff starts equal to the vm_pgoff of the mmap being split?

It is roughly equivalent.  The pgoff in struct map is stored in bytes not in pages, so it doesn't include the shift.

An earlier version of this patch used:
  			after->start = map->end;
+			after->pgoff += map->end - pos->start;

Instead of the newer Functionally equivalent:
  			after->start = map->end;
+			after->pgoff = pos->map_ip(pos, map->end);

I preferred the latter form as it made more sense with the assertion that the mapping of map->end should match in pos and after.

Steve