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References: <20220524234531.1949-1-peterx@redhat.com>
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From:   Guo Ren <guoren@kernel.org>
Date:   Fri, 27 May 2022 10:54:49 +0800
Message-ID: <CAJF2gTTnW6jP9ZQaw-YkhKGWrkH4yTwuryMK=dEVbY0Tup6gMg@mail.gmail.com>
Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared
 memory types
To:     Peter Xu <peterx@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

For csky part.

Acked-by: Guo Ren <guoren@kernel.org>

On Wed, May 25, 2022 at 7:45 AM Peter Xu <peterx@redhat.com> wrote:
>
> 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.
>
> Signed-off-by: Peter Xu <peterx@redhat.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;
>                 }
>         }
>
> --
> 2.32.0
>


-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/