> From: KAMEZAWA Hiroyuki [mailto:kamezawa.hiroyu@xxxxxxxxxxxxxx] > Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache) > > On Thu, 26 Jan 2012 21:15:16 -0800 (PST) > Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote: > > > > From: Rik van Riel [mailto:riel@xxxxxxxxxx] > > > Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache) > > > > > > On 01/26/2012 09:43 PM, Dan Magenheimer wrote: > > > > > > > Maybe the Active page bit could be overloaded with some minor > > > > rewriting? IOW, perhaps the Active bit could be ignored when > > > > the page is moved to the inactive LRU? (Confusing I know, but I am > > > > just brainstorming...) > > > > > > The PG_referenced bit is already overloaded. We keep > > > the bit set when we move a page from the active to the > > > inactive list, so a page that was previously active > > > only needs to be referenced once to become active again. > > > > > > The LRU bits (PG_lru, PG_active, etc) are needed to > > > figure out which LRU list the page is on. I don't > > > think we can overload those... > > > > I suspected that was true, but was just brainstorming. > > Thanks for confirming. > > > > Are there any other page bits that are dont-care when > > a page is on an LRU list? > > How about replacing PG_slab ? > > I think PageSlab(page) be implemented as > > #define SLABMAGIC (some value) > #define PageSlab(page) (page->mapping == SLABMAGIC) > > or some... Hi Kame -- Sounds like a great idea! It looks like the PG_slab bit is part of the kernel<->user ABI (see fs/proc/page.c: stable_page_flags()) but I think it can be simulated without actually using the physical bit in struct pageflags. If so, PG_slab is completely free to be used/overloaded! Here's a possible patch... compile/boot tested but nothing else (and memory-failure.c isn't even compiled and may need more work): diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index dee29fa..ef8498e 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -80,7 +80,7 @@ enum pageflags { PG_dirty, PG_lru, PG_active, - PG_slab, + PG_slab, /* for legacy kernel<->user ABI only */ PG_owner_priv_1, /* Owner use. If pagecache, fs may use*/ PG_arch_1, PG_reserved, @@ -206,7 +206,6 @@ PAGEFLAG(Dirty, dirty) TESTSCFLAG(Dirty, dirty) __CLEARPAGEFLAG(Dirty, dirty) PAGEFLAG(LRU, lru) __CLEARPAGEFLAG(LRU, lru) PAGEFLAG(Active, active) __CLEARPAGEFLAG(Active, active) TESTCLEARFLAG(Active, active) -__PAGEFLAG(Slab, slab) PAGEFLAG(Checked, checked) /* Used by some filesystems */ PAGEFLAG(Pinned, pinned) TESTSCFLAG(Pinned, pinned) /* Xen */ PAGEFLAG(SavePinned, savepinned); /* Xen */ @@ -220,6 +219,28 @@ PAGEFLAG(WasActive, was_active) #endif /* + * for legacy ABI purposes, PG_slab remains defined but all attempted + * uses of the bit are now simulated without using the actual page-flag bit + */ +struct address_space; +#define SLAB_MAGIC ((struct address_space *)0x80758075) +static inline bool PageSlab(struct page *page) +{ + return page->mapping == SLAB_MAGIC; +} + +static inline void __SetPageSlab(struct page *page) +{ + page->mapping = SLAB_MAGIC; +} + +static inline void __ClearPageSlab(struct page *page) +{ + page->mapping = NULL; +} + + +/* * Private page markings that may be used by the filesystem that owns the page * for its own purposes. * - PG_private and PG_private_2 cause releasepage() and co to be invoked diff --git a/mm/memory-failure.c b/mm/memory-failure.c index 06d3479..b4dde77 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -742,7 +742,6 @@ static int me_huge_page(struct page *p, unsigned long pfn) #define head (1UL << PG_head) #define tail (1UL << PG_tail) #define compound (1UL << PG_compound) -#define slab (1UL << PG_slab) #define reserved (1UL << PG_reserved) static struct page_state { @@ -757,13 +756,6 @@ static struct page_state { * PG_buddy pages only make a small fraction of all free pages. */ - /* - * Could in theory check if slab page is free or if we can drop - * currently unused objects without touching them. But just - * treat it as standard kernel for now. - */ - { slab, slab, "kernel slab", me_kernel }, - #ifdef CONFIG_PAGEFLAGS_EXTENDED { head, head, "huge", me_huge_page }, { tail, tail, "huge", me_huge_page }, diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 2b8ba3a..48451a5 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -5787,11 +5787,12 @@ static struct trace_print_flags pageflag_names[] = { {-1UL, NULL }, }; -static void dump_page_flags(unsigned long flags) +static void dump_page_flags(struct page *page) { const char *delim = ""; unsigned long mask; int i; + unsigned long flags = page->flags; printk(KERN_ALERT "page flags: %#lx(", flags); @@ -5801,7 +5802,10 @@ static void dump_page_flags(unsigned long flags) for (i = 0; pageflag_names[i].name && flags; i++) { mask = pageflag_names[i].mask; - if ((flags & mask) != mask) + if (mask == PG_slab) { + if (!PageSlab(page)) + continue; + } else if ((flags & mask) != mask) continue; flags &= ~mask; @@ -5822,6 +5826,6 @@ void dump_page(struct page *page) "page:%p count:%d mapcount:%d mapping:%p index:%#lx\n", page, atomic_read(&page->_count), page_mapcount(page), page->mapping, page->index); - dump_page_flags(page->flags); + dump_page_flags(page); mem_cgroup_print_bad_page(page); } diff --git a/mm/slub.c b/mm/slub.c index ed3334d..a0fdca1 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1361,7 +1361,7 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node) inc_slabs_node(s, page_to_nid(page), page->objects); page->slab = s; - page->flags |= 1 << PG_slab; + page->mapping = SLAB_MAGIC; start = page_address(page); -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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