mm: Avoid unnecessary page fault retires on shared memory types

I observed that for each of the shared file-backed page faults, we're very
likely to retry one more time for the 1st write fault upon no page.  It's
because we'll need to release the mmap lock for dirty rate limit purpose
with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()).

Then after that throttling we return VM_FAULT_RETRY.

We did that probably because VM_FAULT_RETRY is the only way we can return
to the fault handler at that time telling it we've released the mmap lock.

However that's not ideal because it's very likely the fault does not need
to be retried at all since the pgtable was well installed before the
throttling, so the next continuous fault (including taking mmap read lock,
walk the pgtable, etc.) could be in most cases unnecessary.

It's not only slowing down page faults for shared file-backed, but also add
more mmap lock contention which is in most cases not needed at all.

To observe this, one could try to write to some shmem page and look at
"pgfault" value in /proc/vmstat, then we should expect 2 counts for each
shmem write simply because we retried, and vm event "pgfault" will capture
that.

To make it more efficient, add a new VM_FAULT_COMPLETED return code just to
show that we've completed the whole fault and released the lock.  It's also
a hint that we should very possibly not need another fault immediately on
this page because we've just completed it.

This patch provides a ~12% perf boost on my aarch64 test VM with a simple
program sequentially dirtying 400MB shmem file being mmap()ed:

  Before: 650980.20 (+-1.94%)
  After:  569396.40 (+-1.38%)

I believe it could help more than that.

We need some special care on GUP and the s390 pgfault handler (for gmap
code before returning from pgfault), the rest changes in the page fault
handlers should be relatively straightforward.

Another thing to mention is that mm_account_fault() does take this new
fault as a generic fault to be accounted, unlike VM_FAULT_RETRY.

I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do
not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping
them as-is.

Signed-off-by: Peter Xu <peterx@redhat.com>

arch/alpha/mm/fault.c

@@ -155,6 +155,10 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/arc/mm/fault.c

@@ -146,6 +146,10 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 		return;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	/*
 	 * Fault retry nuances, mmap_lock already relinquished by core mm
 	 */

arch/arm/mm/fault.c

@@ -322,6 +322,10 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return 0;
+
 	if (!(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_RETRY) {
 			flags |= FAULT_FLAG_TRIED;

arch/arm64/mm/fault.c

@@ -607,6 +607,10 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
 		return 0;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return 0;
+
 	if (fault & VM_FAULT_RETRY) {
 		mm_flags |= FAULT_FLAG_TRIED;
 		goto retry;

arch/csky/mm/fault.c

@@ -285,6 +285,10 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 		return;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely((fault & VM_FAULT_RETRY) && (flags & FAULT_FLAG_ALLOW_RETRY))) {
 		flags |= FAULT_FLAG_TRIED;
 

arch/hexagon/mm/vm_fault.c

@@ -96,6 +96,10 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	/* The most common case -- we are done. */
 	if (likely(!(fault & VM_FAULT_ERROR))) {
 		if (fault & VM_FAULT_RETRY) {

arch/ia64/mm/fault.c

@@ -139,6 +139,10 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		/*
 		 * We ran out of memory, or some other thing happened

arch/m68k/mm/fault.c

@@ -141,6 +141,10 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (fault_signal_pending(fault, regs))
 		return 0;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return 0;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/microblaze/mm/fault.c

@@ -222,6 +222,10 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/mips/mm/fault.c

@@ -162,6 +162,10 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 		return;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/nios2/mm/fault.c

@@ -139,6 +139,10 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/openrisc/mm/fault.c

@@ -167,6 +167,10 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/parisc/mm/fault.c

@@ -309,6 +309,10 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		/*
 		 * We hit a shared mapping outside of the file, or some

arch/powerpc/mm/copro_fault.c

@@ -65,6 +65,11 @@ int copro_handle_mm_fault(struct mm_struct *mm, unsigned long ea,
 
 	ret = 0;
 	*flt = handle_mm_fault(vma, ea, is_write ? FAULT_FLAG_WRITE : 0, NULL);
+
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (*flt & VM_FAULT_COMPLETED)
+		return 0;
+
 	if (unlikely(*flt & VM_FAULT_ERROR)) {
 		if (*flt & VM_FAULT_OOM) {
 			ret = -ENOMEM;

arch/powerpc/mm/fault.c

@@ -511,6 +511,10 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (fault_signal_pending(fault, regs))
 		return user_mode(regs) ? 0 : SIGBUS;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		goto out;
+
 	/*
 	 * Handle the retry right now, the mmap_lock has been released in that
 	 * case.
@@ -525,6 +529,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (unlikely(fault & VM_FAULT_ERROR))
 		return mm_fault_error(regs, address, fault);
 
+out:
 	/*
 	 * Major/minor page fault accounting.
 	 */

arch/riscv/mm/fault.c

@@ -326,6 +326,10 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_RETRY)) {
 		flags |= FAULT_FLAG_TRIED;
 

arch/s390/mm/fault.c

@@ -339,6 +339,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	unsigned long address;
 	unsigned int flags;
 	vm_fault_t fault;
+	bool need_unlock = true;
 	bool is_write;
 
 	tsk = current;
@@ -433,6 +434,13 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 			goto out_up;
 		goto out;
 	}
+
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED) {
+		need_unlock = false;
+		goto out_gmap;
+	}
+
 	if (unlikely(fault & VM_FAULT_ERROR))
 		goto out_up;
 
@@ -452,6 +460,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 		mmap_read_lock(mm);
 		goto retry;
 	}
+out_gmap:
 	if (IS_ENABLED(CONFIG_PGSTE) && gmap) {
 		address =  __gmap_link(gmap, current->thread.gmap_addr,
 				       address);
@@ -466,7 +475,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	}
 	fault = 0;
 out_up:
-	mmap_read_unlock(mm);
+	if (need_unlock)
+		mmap_read_unlock(mm);
 out:
 	return fault;
 }

arch/sh/mm/fault.c

@@ -485,6 +485,10 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 		if (mm_fault_error(regs, error_code, address, fault))
 			return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (fault & VM_FAULT_RETRY) {
 		flags |= FAULT_FLAG_TRIED;
 

arch/sparc/mm/fault_32.c

@@ -190,6 +190,10 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	if (fault_signal_pending(fault, regs))
 		return;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;
@@ -339,6 +343,9 @@ static void force_user_fault(unsigned long address, int write)
 	case VM_FAULT_OOM:
 		goto do_sigbus;
 	}
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
 	mmap_read_unlock(mm);
 	return;
 bad_area:

arch/sparc/mm/fault_64.c

@@ -427,6 +427,10 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 	if (fault_signal_pending(fault, regs))
 		goto exit_exception;
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		goto lock_released;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;
@@ -449,6 +453,7 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 	}
 	mmap_read_unlock(mm);
 
+lock_released:
 	mm_rss = get_mm_rss(mm);
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE)
 	mm_rss -= (mm->context.thp_pte_count * (HPAGE_SIZE / PAGE_SIZE));

arch/um/kernel/trap.c

@@ -76,6 +76,10 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
 			goto out_nosemaphore;
 
+		/* The fault is fully completed (including releasing mmap lock) */
+		if (fault & VM_FAULT_COMPLETED)
+			return 0;
+
 		if (unlikely(fault & VM_FAULT_ERROR)) {
 			if (fault & VM_FAULT_OOM) {
 				goto out_of_memory;

arch/x86/mm/fault.c

@@ -1408,6 +1408,10 @@ void do_user_addr_fault(struct pt_regs *regs,
 		return;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	/*
 	 * If we need to retry the mmap_lock has already been released,
 	 * and if there is a fatal signal pending there is no guarantee

arch/xtensa/mm/fault.c

@@ -172,6 +172,10 @@ void do_page_fault(struct pt_regs *regs)
 		return;
 	}
 
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

include/linux/mm_types.h

@@ -729,6 +729,7 @@ typedef __bitwise unsigned int vm_fault_t;
  * @VM_FAULT_NEEDDSYNC:		->fault did not modify page tables and needs
  *				fsync() to complete (for synchronous page faults
  *				in DAX)
+ * @VM_FAULT_COMPLETED:		->fault completed, meanwhile mmap lock released
  * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
  *
  */
@@ -746,6 +747,7 @@ enum vm_fault_reason {
 	VM_FAULT_FALLBACK       = (__force vm_fault_t)0x000800,
 	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
 	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
+	VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
 	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
 };
 

mm/gup.c

@@ -951,6 +951,25 @@ static int faultin_page(struct vm_area_struct *vma,
 	}
 
 	ret = handle_mm_fault(vma, address, fault_flags, NULL);
+
+	if (ret & VM_FAULT_COMPLETED) {
+		/*
+		 * With FAULT_FLAG_RETRY_NOWAIT we'll never release the
+		 * mmap lock in the page fault handler. Sanity check this.
+		 */
+		WARN_ON_ONCE(fault_flags & FAULT_FLAG_RETRY_NOWAIT);
+		if (locked)
+			*locked = 0;
+		/*
+		 * We should do the same as VM_FAULT_RETRY, but let's not
+		 * return -EBUSY since that's not reflecting the reality on
+		 * what has happened - we've just fully completed a page
+		 * fault, with the mmap lock released.  Use -EAGAIN to show
+		 * that we want to take the mmap lock _again_.
+		 */
+		return -EAGAIN;
+	}
+
 	if (ret & VM_FAULT_ERROR) {
 		int err = vm_fault_to_errno(ret, *flags);
 
@@ -1177,6 +1196,7 @@ static long __get_user_pages(struct mm_struct *mm,
 			case 0:
 				goto retry;
 			case -EBUSY:
+			case -EAGAIN:
 				ret = 0;
 				fallthrough;
 			case -EFAULT:
@@ -1303,6 +1323,18 @@ int fixup_user_fault(struct mm_struct *mm,
 		return -EINTR;
 
 	ret = handle_mm_fault(vma, address, fault_flags, NULL);
+
+	if (ret & VM_FAULT_COMPLETED) {
+		/*
+		 * NOTE: it's a pity that we need to retake the lock here
+		 * to pair with the unlock() in the callers. Ideally we
+		 * could tell the callers so they do not need to unlock.
+		 */
+		mmap_read_lock(mm);
+		*unlocked = true;
+		return 0;
+	}
+
 	if (ret & VM_FAULT_ERROR) {
 		int err = vm_fault_to_errno(ret, 0);
 
@@ -1368,7 +1400,7 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 			/* VM_FAULT_RETRY couldn't trigger, bypass */
 			return ret;
 
-		/* VM_FAULT_RETRY cannot return errors */
+		/* VM_FAULT_RETRY or VM_FAULT_COMPLETED cannot return errors */
 		if (!*locked) {
 			BUG_ON(ret < 0);
 			BUG_ON(ret >= nr_pages);

mm/memory.c

@@ -3036,7 +3036,7 @@ static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf)
 		balance_dirty_pages_ratelimited(mapping);
 		if (fpin) {
 			fput(fpin);
-			return VM_FAULT_RETRY;
+			return VM_FAULT_COMPLETED;
 		}
 	}