Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

[Vlastimil Babka <vbabka@suse.cz>]

On 8/9/19 12:59 AM, John Hubbard wrote:
>>> That's true. However, I'm not sure munlocking is where the
>>> put_user_page() machinery is intended to be used anyway? These are
>>> short-term pins for struct page manipulation, not e.g. dirtying of page
>>> contents. Reading commit fc1d8e7cca2d I don't think this case falls
>>> within the reasoning there. Perhaps not all GUP users should be
>>> converted to the planned separate GUP tracking, and instead we should
>>> have a GUP/follow_page_mask() variant that keeps using get_page/put_page?
>>>  
>>
>> Interesting. So far, the approach has been to get all the gup callers to
>> release via put_user_page(), but if we add in Jan's and Ira's vaddr_pin_pages()
>> wrapper, then maybe we could leave some sites unconverted.
>>
>> However, in order to do so, we would have to change things so that we have
>> one set of APIs (gup) that do *not* increment a pin count, and another set
>> (vaddr_pin_pages) that do. 
>>
>> Is that where we want to go...?
>>

We already have a FOLL_LONGTERM flag, isn't that somehow related? And if
it's not exactly the same thing, perhaps a new gup flag to distinguish
which kind of pinning to use?

> Oh, and meanwhile, I'm leaning toward a cheap fix: just use gup_fast() instead
> of get_page(), and also fix the releasing code. So this incremental patch, on
> top of the existing one, should do it:
> 
...
> @@ -411,7 +409,13 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
>                 if (PageTransCompound(page))
>                         break;
>  
> -               get_page(page);
> +               /*
> +                * Use get_user_pages_fast(), instead of get_page() so that the
> +                * releasing code can unconditionally call put_user_page().
> +                */
> +               ret = get_user_pages_fast(start, 1, 0, &page);

Um the whole reason of __munlock_pagevec_fill() was to avoid the full
page walk cost, which made a 14% difference, see 7a8010cd3627 ("mm:
munlock: manual pte walk in fast path instead of follow_page_mask()")
Replacing simple get_page() with page walk to satisfy API requirements
seems rather suboptimal to me.

> +               if (ret != 1)
> +                       break;
>                 /*
>                  * Increase the address that will be returned *before* the
>                  * eventual break due to pvec becoming full by adding the page
> 
> 
> thanks,
>