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SubjectRe: [PATCH 3/4] perf symbols: Update kcore map before merging in remaining symbols
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On 5/8/2024 10:14 AM, James Clark wrote:

[...]

>> Looks like you and Leo are working on the same problem.
>>
>> https://lore.kernel.org/r/20240505202805.583253-1-leo.yan@arm.com/
>>
>
> Oops I should have checked the list. It looks like we can still take his
> fix as well though, with an updated comment.

Sorry for duplicate work. I will resend my patch separately with refined
comment, as suggested by Adrian.

[...]

>>> @@ -1289,7 +1289,7 @@ static int dso__load_kcore(struct dso *dso, struct map *map,
>>> {
>>> struct maps *kmaps = map__kmaps(map);
>>> struct kcore_mapfn_data md;
>>> - struct map *replacement_map = NULL;
>>> + struct map *map_ref, *replacement_map = NULL;
>>> struct machine *machine;
>>> bool is_64_bit;
>>> int err, fd;
>>> @@ -1367,6 +1367,24 @@ static int dso__load_kcore(struct dso *dso, struct map *map,
>>> if (!replacement_map)
>>> replacement_map = list_entry(md.maps.next, struct map_list_node, node)->map;

As the 'replacement' map is mainly used to adjust the kernel's sections
between '_stext' and '_end', some arches might don't share the same issue with
Arm64. So it is a bit redundant for assignment 'replacement_map' if it is
NULL, we can consider to remove the above two lines.

>>>
>>> + /*
>>> + * Update addresses of vmlinux map. Re-insert it to ensure maps are
>>> + * correctly ordered. Do this before using maps__merge_in() for the
>>> + * remaining maps so vmlinux gets split if necessary.
>>> + */
>>> + map_ref = map__get(map);
>>> + maps__remove(kmaps, map_ref);
>>
>> A nitpick. It'd be natural to use 'map' instead of 'map_ref'
>> (even if they are the same) since IIUC we want to remove
>> the old 'map' and update 'map_ref' then add it back.
>>
>
> Using map makes sense, I can update that.
>
>>> +
>>> + map__set_start(map_ref, map__start(replacement_map));
>>> + map__set_end(map_ref, map__end(replacement_map));
>>> + map__set_pgoff(map_ref, map__pgoff(replacement_map));
>>> + map__set_mapping_type(map_ref, map__mapping_type(replacement_map));
>>
>> So here, replacement_map should not be NULL right?
>>
>
> Yes it shouldn't be. It would only be NULL if md.maps is empty, but
> there's already an exit condition for that above.
>
> Some of the other code also assumes node->map is always set, so it can't
> be NULL that way either.

Thus, we can consider to check condition for 'replacement' map is NULL or not.

if (replacement_map) {
list_del_init(&new_node->node);

map_ref = map__get(map);
maps__remove(kmaps, map_ref);
...
map__put(new_map);
if (err)
goto out_err;
free(new_node);
}

[...]

>>> @@ -1374,24 +1392,8 @@ static int dso__load_kcore(struct dso *dso, struct map *map,
>>>
>>> list_del_init(&new_node->node);
>>>
>>> - if (RC_CHK_EQUAL(new_map, replacement_map)) {
>>> - struct map *map_ref;
>>> -
>>> - /* Ensure maps are correctly ordered */
>>> - map_ref = map__get(map);
>>> - maps__remove(kmaps, map_ref);
>>> -
>>> - map__set_start(map_ref, map__start(new_map));
>>> - map__set_end(map_ref, map__end(new_map));
>>> - map__set_pgoff(map_ref, map__pgoff(new_map));
>>> - map__set_mapping_type(map_ref, map__mapping_type(new_map));
>>> -
>>> - err = maps__insert(kmaps, map_ref);
>>> - map__put(map_ref);
>>> - map__put(new_map);
>>> - if (err)
>>> - goto out_err;
>>> - } else {
>>> + /* skip if replacement_map, already inserted above */
>>> + if (!RC_CHK_EQUAL(new_map, replacement_map)) {

With above change, we don't need check 'replacement_map' at here.

Just extend a bit for considering a more clean fixing, we need to sort all
ranges in 'md.maps', this would be benefit for two things:

- We can fix up any map regions, not only limit to the 'replacement_map'. With
sorting maps in 'md.maps', we can totally remove the code for
'replacement_map'.
- We can report the potential issue caused by overlapping in the first place
rather than the assert log in check_invariants(). This is easier for later
debugging.

But current patch is good enough for me, I don't have strong opinion for this.

Thanks,
Leo

>>> /*
>>> * Merge kcore map into existing maps,
>>> * and ensure that current maps (eBPF)
>>> --
>>> 2.34.1
>>>

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