Re: [PATCH 7/8] mm: Document how to use struct page

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On Sat 16-12-17 08:44:24, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> 
> Be really explicit about what bits / bytes are reserved for users that
> want to store extra information about the pages they allocate.
> 
> Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

I think that struct page would benefit from more documentation. But this
looks good to me already. Hugetlb pages abuse some fields in page[1],
page_is_pfmemalloc is abusing index and there are probably more. It
would be great to have all those described at the single place. I will
update hugetlb part along with my recent patches which are in RFC right
now. Maybe a good project for somebody who wants to learn a lot about MM
and interaction with other subsystems (or maybe not ;))

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/linux/mm_types.h | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1a3ba1f1605d..a517d210f177 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -31,7 +31,28 @@ struct hmm;
>   * it to keep track of whatever it is we are using the page for at the
>   * moment. Note that we have no way to track which tasks are using
>   * a page, though if it is a pagecache page, rmap structures can tell us
> - * who is mapping it.
> + * who is mapping it. If you allocate the page using alloc_pages(), you
> + * can use some of the space in struct page for your own purposes.
> + *
> + * Pages that were once in the page cache may be found under the RCU lock
> + * even after they have been recycled to a different purpose.  The page cache
> + * will read and writes some of the fields in struct page to lock the page,
> + * then check that it's still in the page cache.  It is vital that all users
> + * of struct page:
> + * 1. Use the first word as PageFlags.
> + * 2. Clear or preserve bit 0 of page->compound_head.  It is used as
> + *    PageTail for compound pages, and the page cache must not see false
> + *    positives.  Some users put a pointer here (guaranteed to be at least
> + *    4-byte aligned), other users avoid using the word altogether.
> + * 3. page->_refcount must either not be used, or must be used in such a
> + *    way that other CPUs temporarily incrementing and then decrementing the
> + *    refcount does not cause problems.  On receiving the page from
> + *    alloc_pages(), the refcount will be positive.
> + *
> + * If you allocate pages of order > 0, you can use the fields in the struct
> + * page associated with each page, but bear in mind that the pages may have
> + * been inserted individually into the page cache, so you must use the above
> + * three fields in a compatible way for each struct page.
>   *
>   * SLUB uses cmpxchg_double() to atomically update its freelist and
>   * counters.  That requires that freelist & counters be adjacent and
> -- 
> 2.15.1
> 
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-- 
Michal Hocko
SUSE Labs

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