[vz8,17/42] mm: allow VM_FAULT_RETRY for multiple times

Submitted by Andrey Ryabinin on April 16, 2020, 10:35 a.m.

Details

Message ID 20200416103620.29930-17-aryabinin@virtuozzo.com
State New
Series "Series without cover letter"
Headers show

Commit Message

Andrey Ryabinin April 16, 2020, 10:35 a.m.
From: Peter Xu <peterx@redhat.com>

The idea comes from a discussion between Linus and Andrea [1].

Before this patch we only allow a page fault to retry once.  We achieved
this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
handle_mm_fault() the second time.  This was majorly used to avoid
unexpected starvation of the system by looping over forever to handle the
page fault on a single page.  However that should hardly happen, and after
all for each code path to return a VM_FAULT_RETRY we'll first wait for a
condition (during which time we should possibly yield the cpu) to happen
before VM_FAULT_RETRY is really returned.

This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY
flag when we receive VM_FAULT_RETRY.  It means that the page fault handler
now can retry the page fault for multiple times if necessary without the
need to generate another page fault event.  Meanwhile we still keep the
FAULT_FLAG_TRIED flag so page fault handler can still identify whether a
page fault is the first attempt or not.

Then we'll have these combinations of fault flags (only considering
ALLOW_RETRY flag and TRIED flag):

  - ALLOW_RETRY and !TRIED:  this means the page fault allows to
                             retry, and this is the first try

  - ALLOW_RETRY and TRIED:   this means the page fault allows to
                             retry, and this is not the first try

  - !ALLOW_RETRY and !TRIED: this means the page fault does not allow
                             to retry at all

  - !ALLOW_RETRY and TRIED:  this is forbidden and should never be used

In existing code we have multiple places that has taken special care of
the first condition above by checking against (fault_flags &
FAULT_FLAG_ALLOW_RETRY).  This patch introduces a simple helper to detect
the first retry of a page fault by checking against both (fault_flags &
FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & FAULT_FLAG_TRIED) because now
even the 2nd try will have the ALLOW_RETRY set, then use that helper in
all existing special paths.  One example is in __lock_page_or_retry(), now
we'll drop the mmap_sem only in the first attempt of page fault and we'll
keep it in follow up retries, so old locking behavior will be retained.

This will be a nice enhancement for current code [2] at the same time a
supporting material for the future userfaultfd-writeprotect work, since in
that work there will always be an explicit userfault writeprotect retry
for protected pages, and if that cannot resolve the page fault (e.g., when
userfaultfd-writeprotect is used in conjunction with swapped pages) then
we'll possibly need a 3rd retry of the page fault.  It might also benefit
other potential users who will have similar requirement like userfault
write-protection.

GUP code is not touched yet and will be covered in follow up patch.

Please read the thread below for more information.

[1] https://lore.kernel.org/lkml/20171102193644.GB22686@redhat.com/
[2] https://lore.kernel.org/lkml/20181230154648.GB9832@redhat.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Brian Geffon <bgeffon@google.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Link: http://lkml.kernel.org/r/20200220160246.9790-1-peterx@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

https://jira.sw.ru/browse/PSBM-102938
(cherry picked from commit 4064b982706375025628094e51d11cf1a958a5d3)
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---
 arch/alpha/mm/fault.c           |  2 +-
 arch/arc/mm/fault.c             |  1 -
 arch/arm/mm/fault.c             |  3 ---
 arch/arm64/mm/fault.c           |  5 -----
 arch/hexagon/mm/vm_fault.c      |  1 -
 arch/ia64/mm/fault.c            |  1 -
 arch/m68k/mm/fault.c            |  3 ---
 arch/microblaze/mm/fault.c      |  1 -
 arch/mips/mm/fault.c            |  1 -
 arch/nds32/mm/fault.c           |  1 -
 arch/nios2/mm/fault.c           |  3 ---
 arch/openrisc/mm/fault.c        |  1 -
 arch/parisc/mm/fault.c          |  4 +---
 arch/powerpc/mm/fault.c         |  6 ------
 arch/riscv/mm/fault.c           |  5 -----
 arch/s390/mm/fault.c            |  5 +----
 arch/sh/mm/fault.c              |  1 -
 arch/sparc/mm/fault_32.c        |  1 -
 arch/sparc/mm/fault_64.c        |  1 -
 arch/um/kernel/trap.c           |  1 -
 arch/unicore32/mm/fault.c       |  4 +---
 arch/x86/mm/fault.c             |  2 --
 arch/xtensa/mm/fault.c          |  1 -
 drivers/gpu/drm/ttm/ttm_bo_vm.c |  7 ++++---
 include/linux/mm.h              | 37 +++++++++++++++++++++++++++++++++
 mm/filemap.c                    |  2 +-
 26 files changed, 46 insertions(+), 54 deletions(-)

Patch hide | download patch | download mbox

diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c
index 7224f53e4127..01d432b2db10 100644
--- a/arch/alpha/mm/fault.c
+++ b/arch/alpha/mm/fault.c
@@ -168,7 +168,7 @@  do_page_fault(unsigned long address, unsigned long mmcsr,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 0f99c591a585..fa565230f026 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -165,7 +165,6 @@  void do_page_fault(unsigned long address, struct pt_regs *regs)
 			}
 
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 				goto retry;
 			}
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 551720493c52..bfc8326bf730 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -343,9 +343,6 @@  do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 					regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 905655337a89..3d1276e81479 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -504,12 +504,7 @@  static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	}
 
 	if (fault & VM_FAULT_RETRY) {
-		/*
-		 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk of
-		 * starvation.
-		 */
 		if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
-			mm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			mm_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
diff --git a/arch/hexagon/mm/vm_fault.c b/arch/hexagon/mm/vm_fault.c
index 106fa57726d1..ab6b756f5ea2 100644
--- a/arch/hexagon/mm/vm_fault.c
+++ b/arch/hexagon/mm/vm_fault.c
@@ -115,7 +115,6 @@  void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 				goto retry;
 			}
diff --git a/arch/ia64/mm/fault.c b/arch/ia64/mm/fault.c
index bdaee2821d6d..8092634f9915 100644
--- a/arch/ia64/mm/fault.c
+++ b/arch/ia64/mm/fault.c
@@ -189,7 +189,6 @@  ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 2c4a6bcd25e4..2b344ffc40ea 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -162,9 +162,6 @@  int do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/microblaze/mm/fault.c b/arch/microblaze/mm/fault.c
index 43f74d3b0dd7..5566171a550b 100644
--- a/arch/microblaze/mm/fault.c
+++ b/arch/microblaze/mm/fault.c
@@ -236,7 +236,6 @@  void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
index 216d0bfe9413..16e0001eae54 100644
--- a/arch/mips/mm/fault.c
+++ b/arch/mips/mm/fault.c
@@ -178,7 +178,6 @@  static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 			tsk->min_flt++;
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/nds32/mm/fault.c b/arch/nds32/mm/fault.c
index fcf776fd8f3a..abb651016e6b 100644
--- a/arch/nds32/mm/fault.c
+++ b/arch/nds32/mm/fault.c
@@ -237,7 +237,6 @@  void do_page_fault(unsigned long entry, unsigned long addr,
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/nios2/mm/fault.c b/arch/nios2/mm/fault.c
index 5ac5f755b934..94dd9c11b569 100644
--- a/arch/nios2/mm/fault.c
+++ b/arch/nios2/mm/fault.c
@@ -158,9 +158,6 @@  asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/openrisc/mm/fault.c b/arch/openrisc/mm/fault.c
index 70e365824529..2aee240e1a60 100644
--- a/arch/openrisc/mm/fault.c
+++ b/arch/openrisc/mm/fault.c
@@ -185,7 +185,6 @@  asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
index 3e51aa5b236e..4da150933fbd 100644
--- a/arch/parisc/mm/fault.c
+++ b/arch/parisc/mm/fault.c
@@ -327,14 +327,12 @@  void do_page_fault(struct pt_regs *regs, unsigned long code,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
-
 			/*
 			 * No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
-
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 115a146024dc..f380ba209fd1 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -565,13 +565,7 @@  static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * case.
 	 */
 	if (unlikely(fault & VM_FAULT_RETRY)) {
-		/* We retry only once */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 55dfa5e57ded..98142a3ccbf7 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -153,11 +153,6 @@  asmlinkage void do_page_fault(struct pt_regs *regs)
 				      1, regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY);
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 9350a8dab503..94380a200782 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -532,10 +532,7 @@  static inline int do_exception(struct pt_regs *regs, int access)
 				fault = VM_FAULT_PFAULT;
 				goto out_up;
 			}
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY |
-				   FAULT_FLAG_RETRY_NOWAIT);
+			flags &= ~FAULT_FLAG_RETRY_NOWAIT;
 			flags |= FAULT_FLAG_TRIED;
 			down_read(&mm->mmap_sem);
 			goto retry;
diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c
index 73f491c290f1..eb583c5f0d84 100644
--- a/arch/sh/mm/fault.c
+++ b/arch/sh/mm/fault.c
@@ -498,7 +498,6 @@  asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 				      regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/sparc/mm/fault_32.c b/arch/sparc/mm/fault_32.c
index 813db4383cff..107ff861efea 100644
--- a/arch/sparc/mm/fault_32.c
+++ b/arch/sparc/mm/fault_32.c
@@ -260,7 +260,6 @@  asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/sparc/mm/fault_64.c b/arch/sparc/mm/fault_64.c
index 8c6de127a641..c590a673df5a 100644
--- a/arch/sparc/mm/fault_64.c
+++ b/arch/sparc/mm/fault_64.c
@@ -458,7 +458,6 @@  asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index f397bffc1631..5750c45d02a4 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -96,7 +96,6 @@  int handle_page_fault(unsigned long address, unsigned long ip,
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 
 				goto retry;
diff --git a/arch/unicore32/mm/fault.c b/arch/unicore32/mm/fault.c
index 743fc037c5ed..575414ff64c7 100644
--- a/arch/unicore32/mm/fault.c
+++ b/arch/unicore32/mm/fault.c
@@ -267,9 +267,7 @@  static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index f171ff2b580c..b59a104f95eb 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1420,8 +1420,6 @@  __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 	 */
 	if (unlikely((fault & VM_FAULT_RETRY) &&
 		     (flags & FAULT_FLAG_ALLOW_RETRY))) {
-		/* Retry at most once */
-		flags &= ~FAULT_FLAG_ALLOW_RETRY;
 		flags |= FAULT_FLAG_TRIED;
 		goto retry;
 	}
diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index 3071dc6f3196..da8e5ec69aeb 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -128,7 +128,6 @@  void do_page_fault(struct pt_regs *regs)
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index c7ece7613a6a..e1622923f372 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -60,9 +60,10 @@  static int ttm_bo_vm_fault_idle(struct ttm_buffer_object *bo,
 
 	/*
 	 * If possible, avoid waiting for GPU with mmap_sem
-	 * held.
+	 * held.  We only do this if the fault allows retry and this
+	 * is the first attempt.
 	 */
-	if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(vmf->flags)) {
 		ret = VM_FAULT_RETRY;
 		if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_unlock;
@@ -134,7 +135,7 @@  static int ttm_bo_vm_fault(struct vm_fault *vmf)
 		if (ret != -EBUSY)
 			return VM_FAULT_NOPAGE;
 
-		if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+		if (fault_flag_allow_retry_first(vmf->flags)) {
 			if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
 				ttm_bo_reference(bo);
 				up_read(&vmf->vma->vm_mm->mmap_sem);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 927059b8d679..9bddcabb654c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -325,6 +325,25 @@  extern pgprot_t protection_map[16];
  * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
  * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
  * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ *
+ * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
+ * whether we would allow page faults to retry by specifying these two
+ * fault flags correctly.  Currently there can be three legal combinations:
+ *
+ * (a) ALLOW_RETRY and !TRIED:  this means the page fault allows retry, and
+ *                              this is the first try
+ *
+ * (b) ALLOW_RETRY and TRIED:   this means the page fault allows retry, and
+ *                              we've already tried at least once
+ *
+ * (c) !ALLOW_RETRY and !TRIED: this means the page fault does not allow retry
+ *
+ * The unlisted combination (!ALLOW_RETRY && TRIED) is illegal and should never
+ * be used.  Note that page faults can be allowed to retry for multiple times,
+ * in which case we'll have an initial fault with flags (a) then later on
+ * continuous faults with flags (b).  We should always try to detect pending
+ * signals before a retry to make sure the continuous page faults can still be
+ * interrupted if necessary.
  */
 #define FAULT_FLAG_WRITE			0x01
 #define FAULT_FLAG_MKWRITE			0x02
@@ -345,6 +364,24 @@  extern pgprot_t protection_map[16];
 			     FAULT_FLAG_KILLABLE | \
 			     FAULT_FLAG_INTERRUPTIBLE)
 
+/**
+ * fault_flag_allow_retry_first - check ALLOW_RETRY the first time
+ *
+ * This is mostly used for places where we want to try to avoid taking
+ * the mmap_sem for too long a time when waiting for another condition
+ * to change, in which case we can try to be polite to release the
+ * mmap_sem in the first round to avoid potential starvation of other
+ * processes that would also want the mmap_sem.
+ *
+ * Return: true if the page fault allows retry and this is the first
+ * attempt of the fault handling; false otherwise.
+ */
+static inline bool fault_flag_allow_retry_first(unsigned int flags)
+{
+	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
+	    (!(flags & FAULT_FLAG_TRIED));
+}
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \
diff --git a/mm/filemap.c b/mm/filemap.c
index dc3c3c5ae6fa..ce09912ca276 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1351,7 +1351,7 @@  EXPORT_SYMBOL_GPL(__lock_page_killable);
 int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 			 unsigned int flags)
 {
-	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(flags)) {
 		/*
 		 * CAUTION! In this case, mmap_sem is not released
 		 * even though return 0.