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 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
