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Message-ID: <87pmjz9yfy.fsf@nvdebian.thelocal>
Date: Fri, 27 May 2022 18:21:37 +1000
From: Alistair Popple <apopple@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Richard Henderson <rth@...ddle.net>,
David Hildenbrand <david@...hat.com>,
Matt Turner <mattst88@...il.com>,
Albert Ou <aou@...s.berkeley.edu>,
Michal Simek <monstr@...str.eu>,
Russell King <linux@...linux.org.uk>,
Ivan Kokshaysky <ink@...assic.park.msu.ru>,
linux-riscv@...ts.infradead.org,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Jonas Bonn <jonas@...thpole.se>, Will Deacon <will@...nel.org>,
"James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
"H . Peter Anvin" <hpa@...or.com>,
Andrea Arcangeli <aarcange@...hat.com>,
openrisc@...ts.librecores.org, linux-s390@...r.kernel.org,
Ingo Molnar <mingo@...hat.com>,
linux-m68k@...ts.linux-m68k.org,
Palmer Dabbelt <palmer@...belt.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Chris Zankel <chris@...kel.net>,
Peter Zijlstra <peterz@...radead.org>,
linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
Vlastimil Babka <vbabka@...e.cz>,
Thomas Gleixner <tglx@...utronix.de>,
sparclinux@...r.kernel.org,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Stafford Horne <shorne@...il.com>,
Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
Paul Mackerras <paulus@...ba.org>,
linux-arm-kernel@...ts.infradead.org,
Sven Schnelle <svens@...ux.ibm.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
linux-xtensa@...ux-xtensa.org, Nicholas Piggin <npiggin@...il.com>,
linux-sh@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
Borislav Petkov <bp@...en8.de>, linux-mips@...r.kernel.org,
Max Filippov <jcmvbkbc@...il.com>,
Helge Deller <deller@....de>, Vineet Gupta <vgupta@...nel.org>,
Al Viro <viro@...iv.linux.org.uk>,
Paul Walmsley <paul.walmsley@...ive.com>,
Johannes Weiner <hannes@...xchg.org>,
Anton Ivanov <anton.ivanov@...bridgegreys.com>,
Catalin Marinas <catalin.marinas@....com>,
linux-um@...ts.infradead.org, linux-alpha@...r.kernel.org,
Johannes Berg <johannes@...solutions.net>,
linux-ia64@...r.kernel.org,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Dinh Nguyen <dinguyen@...nel.org>, Guo Ren <guoren@...nel.org>,
linux-snps-arc@...ts.infradead.org,
Hugh Dickins <hughd@...gle.com>, Rich Felker <dalias@...c.org>,
Andy Lutomirski <luto@...nel.org>,
Richard Weinberger <richard@....at>,
linuxppc-dev@...ts.ozlabs.org, Brian Cain <bcain@...cinc.com>,
Yoshinori Sato <ysato@...rs.sourceforge.jp>,
Andrew Morton <akpm@...ux-foundation.org>,
Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
linux-parisc@...r.kernel.org,
"David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared
memory types
Peter Xu <peterx@...hat.com> writes:
> 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 and these are
> the time it needs:
>
> Before: 650.980 ms (+-1.94%)
> After: 569.396 ms (+-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.
I looked at the change generally and in particular for hmm_vma_fault()
and didn't see any issues there so feel free to add:
Reviewed-by: Alistair Popple <apopple@...dia.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>
> v3:
> - Rebase to akpm/mm-unstable
> - Copy arch maintainers
> ---
> arch/alpha/mm/fault.c | 4 ++++
> arch/arc/mm/fault.c | 4 ++++
> arch/arm/mm/fault.c | 4 ++++
> arch/arm64/mm/fault.c | 4 ++++
> arch/csky/mm/fault.c | 4 ++++
> arch/hexagon/mm/vm_fault.c | 4 ++++
> arch/ia64/mm/fault.c | 4 ++++
> arch/m68k/mm/fault.c | 4 ++++
> arch/microblaze/mm/fault.c | 4 ++++
> arch/mips/mm/fault.c | 4 ++++
> arch/nios2/mm/fault.c | 4 ++++
> arch/openrisc/mm/fault.c | 4 ++++
> arch/parisc/mm/fault.c | 4 ++++
> arch/powerpc/mm/copro_fault.c | 5 +++++
> arch/powerpc/mm/fault.c | 5 +++++
> arch/riscv/mm/fault.c | 4 ++++
> arch/s390/mm/fault.c | 12 +++++++++++-
> arch/sh/mm/fault.c | 4 ++++
> arch/sparc/mm/fault_32.c | 4 ++++
> arch/sparc/mm/fault_64.c | 5 +++++
> arch/um/kernel/trap.c | 4 ++++
> arch/x86/mm/fault.c | 4 ++++
> arch/xtensa/mm/fault.c | 4 ++++
> include/linux/mm_types.h | 2 ++
> mm/gup.c | 34 +++++++++++++++++++++++++++++++++-
> mm/memory.c | 2 +-
> 26 files changed, 138 insertions(+), 3 deletions(-)
>
> diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c
> index ec20c1004abf..ef427a6bdd1a 100644
> --- a/arch/alpha/mm/fault.c
> +++ b/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;
> diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
> index dad27e4d69ff..5ca59a482632 100644
> --- a/arch/arc/mm/fault.c
> +++ b/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
> */
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index a062e07516dd..46cccd6bf705 100644
> --- a/arch/arm/mm/fault.c
> +++ b/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;
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 77341b160aca..e401d416bbd6 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/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;
> diff --git a/arch/csky/mm/fault.c b/arch/csky/mm/fault.c
> index 7215a46b6b8e..e15f736cca4b 100644
> --- a/arch/csky/mm/fault.c
> +++ b/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;
>
> diff --git a/arch/hexagon/mm/vm_fault.c b/arch/hexagon/mm/vm_fault.c
> index 4fac4b9eb316..f73c7cbfe326 100644
> --- a/arch/hexagon/mm/vm_fault.c
> +++ b/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) {
> diff --git a/arch/ia64/mm/fault.c b/arch/ia64/mm/fault.c
> index 07379d1a227f..ef78c2d66cdd 100644
> --- a/arch/ia64/mm/fault.c
> +++ b/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
> diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
> index 71aa9f6315dc..4d2837eb3e2a 100644
> --- a/arch/m68k/mm/fault.c
> +++ b/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;
> diff --git a/arch/microblaze/mm/fault.c b/arch/microblaze/mm/fault.c
> index a9626e6a68af..5c40c3ebe52f 100644
> --- a/arch/microblaze/mm/fault.c
> +++ b/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;
> diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
> index 44f98100e84e..6f72bac39bf2 100644
> --- a/arch/mips/mm/fault.c
> +++ b/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;
> diff --git a/arch/nios2/mm/fault.c b/arch/nios2/mm/fault.c
> index a32f14cd72f2..edaca0a6c1c1 100644
> --- a/arch/nios2/mm/fault.c
> +++ b/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;
> diff --git a/arch/openrisc/mm/fault.c b/arch/openrisc/mm/fault.c
> index 80bb66ad42f6..c18f7abd64df 100644
> --- a/arch/openrisc/mm/fault.c
> +++ b/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;
> diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
> index f114e102aaf2..fe57175a7792 100644
> --- a/arch/parisc/mm/fault.c
> +++ b/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
> diff --git a/arch/powerpc/mm/copro_fault.c b/arch/powerpc/mm/copro_fault.c
> index c1cb21a00884..7c507fb48182 100644
> --- a/arch/powerpc/mm/copro_fault.c
> +++ b/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;
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index d53fed4eccbd..014005428687 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/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.
> */
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index 4e9efbe46d5f..d6a87f4137ca 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -330,6 +330,10 @@ 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;
>
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index e173b6187ad5..9503a7cfaf03 100644
> --- a/arch/s390/mm/fault.c
> +++ b/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;
> }
> diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c
> index e175667b1363..acd2f5e50bfc 100644
> --- a/arch/sh/mm/fault.c
> +++ b/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;
>
> diff --git a/arch/sparc/mm/fault_32.c b/arch/sparc/mm/fault_32.c
> index ad569d9bd124..91259f291c54 100644
> --- a/arch/sparc/mm/fault_32.c
> +++ b/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;
> diff --git a/arch/sparc/mm/fault_64.c b/arch/sparc/mm/fault_64.c
> index 253e07043298..4acc12eafbf5 100644
> --- a/arch/sparc/mm/fault_64.c
> +++ b/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));
> diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
> index d1d5d0be0308..d3ce21c4ca32 100644
> --- a/arch/um/kernel/trap.c
> +++ b/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;
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index d0074c6ed31a..12ed70b432d6 100644
> --- a/arch/x86/mm/fault.c
> +++ b/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
> diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
> index 06d0973a0d74..5f64305ba8d7 100644
> --- a/arch/xtensa/mm/fault.c
> +++ b/arch/xtensa/mm/fault.c
> @@ -118,6 +118,10 @@ 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;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index c09b7f0555b8..decc275db3c9 100644
> --- a/include/linux/mm_types.h
> +++ b/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,
> };
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2e07cff3b31b..3347b083d70b 100644
> --- a/mm/gup.c
> +++ b/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);
> diff --git a/mm/memory.c b/mm/memory.c
> index 54d106e0c999..a8be2d7a8718 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3020,7 +3020,7 @@ 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;
> }
> }
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