linux-kernel - Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user

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Message-ID: <420a5039-a79c-3872-38ea-807cedca3b8a@suse.cz>
Date:   Fri, 9 Aug 2019 10:12:48 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerome Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Black <daniel@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

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,
>