Re: [GIT PULL] hotfixes for 6.3-rc1

[Huang, Ying]

Vlastimil Babka <vbabka@xxxxxxx> writes:

> On 3/6/23 02:25, Huang, Ying wrote:
>> Hi, Linus,
>> 
>> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
>> 
>>> On Sat, Mar 4, 2023 at 3:21 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> Ah. Ying did it this way:
>>>
>>> Yeah, I saw that patch flying past, but I actually think that it only
>>> silences the warning almost by mistake. There's nothing fundamental in
>>> there that a compiler wouldn't just follow across two assignments, and
>>> it just happens to now not trigger any more.
>>>
>>> Assigning to a union entry is a more fundamental operation in that
>>> respect. Not that the compiler still doesn't see that it's assigning a
>>> value that in the end is not really type compatible, so a different
>>> version of gcc could still warn, but at that point I feel like it's
>>> more of an actual compiler bug than just "oh, the compiler didn't
>>> happen to follow the cast through a temporary".
>> 
>> Yes.  Your fix is much better.  This can be used for
>> __page_set_anon_rmap() family too to make the code look better?
>
> Those are trickier due to the PAGE_MAPPING_ANON tagging bit which your
> code doesn't need to handle because you can simply store an untagged
> anon_vma pointer. Otherwise we could have the union at the struct page
> level already (but probably not at this point as IIRC Matthew is
> planning to have completely separate types for anon and file folios).
>
> So right now we have e.g. folio_get_anon_vma() using unsigned long as
> the intermediate variable, page_move_anon_rmap() using a void *
> variable, __page_set_anon_rmap() casting through a (void *) ... Is there
> a single recommended way for "tagged pointers" that we could unify that to?

Ah, you are right.  We need to deal with tagging bit and maybe struct
movable_operations *.  I tried to write the below debug patch (only
build test it).  The code adds 1 or 2 lines for each function.  But to
be honest, the original force casting method appears more natural to me.

Best Regards,
Huang, Ying

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