Date: Tue, 21 Jun 2011 16:11:52 -0600 From: Warner Losh <imp@bsdimp.com> To: "Bjoern A. Zeeb" <bz@freebsd.org> Cc: Alan Cox <alc@freebsd.org>, svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r223307 - head/sys/vm Message-ID: <4A0467C1-258F-4D8D-91E0-9CD1795A49B6@bsdimp.com> In-Reply-To: <BBC34F79-FFA7-4A05-83B3-DE17E0AB14D0@FreeBSD.org> References: <201106191913.p5JJDOqJ006272@svn.freebsd.org> <BBC34F79-FFA7-4A05-83B3-DE17E0AB14D0@FreeBSD.org>
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On Jun 21, 2011, at 2:30 PM, Bjoern A. Zeeb wrote: > On Jun 19, 2011, at 7:13 PM, Alan Cox wrote: > > Hi Alan, > >> Author: alc >> Date: Sun Jun 19 19:13:24 2011 >> New Revision: 223307 >> URL: http://svn.freebsd.org/changeset/base/223307 >> >> Log: >> Precisely document the synchronization rules for the page's dirty field. >> (Saying that the lock on the object that the page belongs to must be held >> only represents one aspect of the rules.) >> >> Eliminate the use of the page queues lock for atomically performing read- >> modify-write operations on the dirty field when the underlying architecture >> supports atomic operations on char and short types. >> >> Document the fact that 32KB pages aren't really supported. >> > > contrary to the tinderbox I'd like to point out that all mips kernels built by universe are broken with a SVN HEAD from earlier today. Could you please check and see if you can fix it? The errors I get are: > > vm_page.o: In function `vm_page_clear_dirty': > /sys/vm/vm_page.c:(.text+0x18d0): undefined reference to `atomic_clear_8' > /sys/vm/vm_page.c:(.text+0x18d0): relocation truncated to fit: R_MIPS_26 against `atomic_clear_8' > vm_page.o: In function `vm_page_set_validclean': > /sys/vm/vm_page.c:(.text+0x38f0): undefined reference to `atomic_clear_8' > /sys/vm/vm_page.c:(.text+0x38f0): relocation truncated to fit: R_MIPS_26 against `atomic_clear_8' atomic_clear_8???? I only see atomic_clear_int, which should be atomic_clear_4 in the mips impl. Warner > Thanks a lot! > /bz > > >> Reviewed by: attilio, kib >> >> Modified: >> head/sys/vm/vm_fault.c >> head/sys/vm/vm_page.c >> head/sys/vm/vm_page.h >> >> Modified: head/sys/vm/vm_fault.c >> ============================================================================== >> --- head/sys/vm/vm_fault.c Sun Jun 19 18:34:49 2011 (r223306) >> +++ head/sys/vm/vm_fault.c Sun Jun 19 19:13:24 2011 (r223307) >> @@ -1089,10 +1089,20 @@ vm_fault_quick_hold_pages(vm_map_t map, >> * caller's changes may go unnoticed because they are >> * performed through an unmanaged mapping or by a DMA >> * operation. >> + * >> + * The object lock is not held here. Therefore, like >> + * a pmap operation, the page queues lock may be >> + * required in order to call vm_page_dirty(). See >> + * vm_page_clear_dirty_mask(). >> */ >> +#if defined(__amd64__) || defined(__i386__) || defined(__ia64__) || \ >> + defined(__mips__) >> + vm_page_dirty(*mp); >> +#else >> vm_page_lock_queues(); >> vm_page_dirty(*mp); >> vm_page_unlock_queues(); >> +#endif >> } >> } >> if (pmap_failed) { >> >> Modified: head/sys/vm/vm_page.c >> ============================================================================== >> --- head/sys/vm/vm_page.c Sun Jun 19 18:34:49 2011 (r223306) >> +++ head/sys/vm/vm_page.c Sun Jun 19 19:13:24 2011 (r223307) >> @@ -729,7 +729,12 @@ vm_page_sleep(vm_page_t m, const char *m >> /* >> * vm_page_dirty: >> * >> - * make page all dirty >> + * Set all bits in the page's dirty field. >> + * >> + * The object containing the specified page must be locked if the call is >> + * made from the machine-independent layer. If, however, the call is >> + * made from the pmap layer, then the page queues lock may be required. >> + * See vm_page_clear_dirty_mask(). >> */ >> void >> vm_page_dirty(vm_page_t m) >> @@ -2325,15 +2330,41 @@ vm_page_clear_dirty_mask(vm_page_t m, in >> /* >> * If the object is locked and the page is neither VPO_BUSY nor >> * PG_WRITEABLE, then the page's dirty field cannot possibly be >> - * modified by a concurrent pmap operation. >> + * set by a concurrent pmap operation. >> */ >> VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED); >> if ((m->oflags & VPO_BUSY) == 0 && (m->flags & PG_WRITEABLE) == 0) >> m->dirty &= ~pagebits; >> else { >> +#if defined(__amd64__) || defined(__i386__) || defined(__ia64__) || \ >> + defined(__mips__) >> + /* >> + * On the aforementioned architectures, the page queues lock >> + * is not required by the following read-modify-write >> + * operation. The combination of the object's lock and an >> + * atomic operation suffice. Moreover, the pmap layer on >> + * these architectures can call vm_page_dirty() without >> + * holding the page queues lock. >> + */ >> +#if PAGE_SIZE == 4096 >> + atomic_clear_char(&m->dirty, pagebits); >> +#elif PAGE_SIZE == 8192 >> + atomic_clear_short(&m->dirty, pagebits); >> +#elif PAGE_SIZE == 16384 >> + atomic_clear_int(&m->dirty, pagebits); >> +#else >> +#error "PAGE_SIZE is not supported." >> +#endif >> +#else >> + /* >> + * Otherwise, the page queues lock is required to ensure that >> + * a concurrent pmap operation does not set the page's dirty >> + * field during the following read-modify-write operation. >> + */ >> vm_page_lock_queues(); >> m->dirty &= ~pagebits; >> vm_page_unlock_queues(); >> +#endif >> } >> } >> >> >> Modified: head/sys/vm/vm_page.h >> ============================================================================== >> --- head/sys/vm/vm_page.h Sun Jun 19 18:34:49 2011 (r223306) >> +++ head/sys/vm/vm_page.h Sun Jun 19 19:13:24 2011 (r223307) >> @@ -89,10 +89,26 @@ >> * and offset to which this page belongs (for pageout), >> * and sundry status bits. >> * >> - * Fields in this structure are locked either by the lock on the >> - * object that the page belongs to (O), its corresponding page lock (P), >> - * or by the lock on the page queues (Q). >> - * >> + * In general, operations on this structure's mutable fields are >> + * synchronized using either one of or a combination of the lock on the >> + * object that the page belongs to (O), the pool lock for the page (P), >> + * or the lock for either the free or paging queues (Q). If a field is >> + * annotated below with two of these locks, then holding either lock is >> + * sufficient for read access, but both locks are required for write >> + * access. >> + * >> + * In contrast, the synchronization of accesses to the page's dirty field >> + * is machine dependent (M). In the machine-independent layer, the lock >> + * on the object that the page belongs to must be held in order to >> + * operate on the field. However, the pmap layer is permitted to set >> + * all bits within the field without holding that lock. Therefore, if >> + * the underlying architecture does not support atomic read-modify-write >> + * operations on the field's type, then the machine-independent layer >> + * must also hold the page queues lock when performing read-modify-write >> + * operations and the pmap layer must hold the page queues lock when >> + * setting the field. In the machine-independent layer, the >> + * implementation of read-modify-write operations on the field is >> + * encapsulated in vm_page_clear_dirty_mask(). >> */ >> >> TAILQ_HEAD(pglist, vm_page); >> @@ -120,18 +136,19 @@ struct vm_page { >> u_char busy; /* page busy count (O) */ >> /* NOTE that these must support one bit per DEV_BSIZE in a page!!! */ >> /* so, on normal X86 kernels, they must be at least 8 bits wide */ >> + /* In reality, support for 32KB pages is not fully implemented. */ >> #if PAGE_SIZE == 4096 >> u_char valid; /* map of valid DEV_BSIZE chunks (O) */ >> - u_char dirty; /* map of dirty DEV_BSIZE chunks (O) */ >> + u_char dirty; /* map of dirty DEV_BSIZE chunks (M) */ >> #elif PAGE_SIZE == 8192 >> u_short valid; /* map of valid DEV_BSIZE chunks (O) */ >> - u_short dirty; /* map of dirty DEV_BSIZE chunks (O) */ >> + u_short dirty; /* map of dirty DEV_BSIZE chunks (M) */ >> #elif PAGE_SIZE == 16384 >> u_int valid; /* map of valid DEV_BSIZE chunks (O) */ >> - u_int dirty; /* map of dirty DEV_BSIZE chunks (O) */ >> + u_int dirty; /* map of dirty DEV_BSIZE chunks (M) */ >> #elif PAGE_SIZE == 32768 >> u_long valid; /* map of valid DEV_BSIZE chunks (O) */ >> - u_long dirty; /* map of dirty DEV_BSIZE chunks (O) */ >> + u_long dirty; /* map of dirty DEV_BSIZE chunks (M) */ >> #endif >> }; >> > > -- > Bjoern A. Zeeb You have to have visions! > Stop bit received. Insert coin for new address family. > > >
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