On 04/08/2018 04:23 PM, Matthew Wilcox wrote: > > Please let me know if this way of expressing the layout of struct page > makes more sense to you. I'm trying to make it easier for other users > to use some of the space in struct page, and without knowing the VM well, > it's hard to know what fields you can safely overload. I like this approach! Tried going all the way and do it for all doublewords? Or why did you leave the third one unchanged? The _refcount seems misplaced there in a struct with _mapcount btw, it's not page cache specific. > --- > > One of the confusing things about trying to use struct page is knowing > which fields are already in use by what. Try and bring some order to > this by grouping the various fields together into sub-structs. Verified > that the layout does not change with pahole. BTW, I recommend you get/build a version of pahole that has this commit: https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=2dd87be78bb23c071708c93f5180c4b94844759c Vlastimil > Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> > --- > include/linux/mm_types.h | 67 ++++++++++++++++++++++-------------------------- > 1 file changed, 31 insertions(+), 36 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 1c5dea402501..97ceec1c6e21 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -78,19 +78,18 @@ struct page { > unsigned long flags; /* Atomic flags, some possibly > * updated asynchronously */ > union { > - /* See page-flags.h for the definition of PAGE_MAPPING_FLAGS */ > - struct address_space *mapping; > - > - void *s_mem; /* slab first object */ > + struct { /* Page cache */ > + /* See page-flags.h for PAGE_MAPPING_FLAGS */ > + struct address_space *mapping; > + pgoff_t index; /* Our offset within mapping. */ > + }; > + struct { /* slab/slob/slub */ > + void *s_mem; /* first object */ > + /* Second dword boundary */ > + void *freelist; /* first free object */ > + }; > atomic_t compound_mapcount; /* first tail page */ > - /* page_deferred_list().next -- second tail page */ > - }; > - > - /* Second double word */ > - union { > - pgoff_t index; /* Our offset within mapping. */ > - void *freelist; /* sl[aou]b first free object */ > - /* page_deferred_list().prev -- second tail page */ > + struct list_head deferred_list; /* second tail page */ > }; > > union { > @@ -132,17 +131,27 @@ struct page { > * avoid collision and false-positive PageTail(). > */ > union { > - struct list_head lru; /* Pageout list, eg. active_list > - * protected by zone_lru_lock ! > - * Can be used as a generic list > - * by the page owner. > - */ > + struct { /* Page cache */ > + /** > + * @lru: Pageout list, eg. active_list protected by > + * zone_lru_lock. Can be used as a generic list by > + * the page owner. > + */ > + struct list_head lru; > + /* > + * Mapping-private opaque data: > + * Usually used for buffer_heads if PagePrivate > + * Used for swp_entry_t if PageSwapCache > + * Indicates order in the buddy system if PageBuddy > + */ > + unsigned long private; > + }; > struct dev_pagemap *pgmap; /* ZONE_DEVICE pages are never on an > * lru or handled by a slab > * allocator, this points to the > * hosting device page map. > */ > - struct { /* slub per cpu partial pages */ > + struct { /* slab/slob/slub */ > struct page *next; /* Next partial slab */ > #ifdef CONFIG_64BIT > int pages; /* Nr of partial slabs left */ > @@ -151,6 +160,7 @@ struct page { > short int pages; > short int pobjects; > #endif > + struct kmem_cache *slab_cache; /* Pointer to slab */ > }; > > struct rcu_head rcu_head; /* Used by SLAB > @@ -166,33 +176,18 @@ struct page { > /* two/six bytes available here */ > }; > > -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS > struct { > unsigned long __pad; /* do not overlay pmd_huge_pte > * with compound_head to avoid > * possible bit 0 collision. > */ > pgtable_t pmd_huge_pte; /* protected by page->ptl */ > - }; > -#endif > - }; > - > - union { > - /* > - * Mapping-private opaque data: > - * Usually used for buffer_heads if PagePrivate > - * Used for swp_entry_t if PageSwapCache > - * Indicates order in the buddy system if PageBuddy > - */ > - unsigned long private; > -#if USE_SPLIT_PTE_PTLOCKS > #if ALLOC_SPLIT_PTLOCKS > - spinlock_t *ptl; > + spinlock_t *ptl; > #else > - spinlock_t ptl; > + spinlock_t ptl; > #endif > -#endif > - struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */ > + }; > }; > > #ifdef CONFIG_MEMCG >
