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([2001:b07:add:ec09:c399:bc87:7b6c:fb2a]) by smtp.googlemail.com with ESMTPSA id g11-20020a170906394b00b007abafe43c3bsm5575689eje.86.2022.11.02.10.06.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Nov 2022 10:06:24 -0700 (PDT) Message-ID: Date: Wed, 2 Nov 2022 18:06:20 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH v4 1/4] mm/gup: Add FOLL_INTERRUPTIBLE Content-Language: en-US To: Peter Xu , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Sean Christopherson , John Hubbard , David Matlack , Andrew Morton , Andrea Arcangeli , "Dr . David Alan Gilbert" , David Hildenbrand , Linux MM Mailing List , Mike Kravetz References: <20221011195809.557016-1-peterx@redhat.com> <20221011195809.557016-2-peterx@redhat.com> From: Paolo Bonzini In-Reply-To: <20221011195809.557016-2-peterx@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/11/22 21:58, Peter Xu wrote: > We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs. One > issue with it is that not all GUP paths are able to handle signal delivers > besides SIGKILL. > > That's not ideal for the GUP users who are actually able to handle these > cases, like KVM. > > KVM uses GUP extensively on faulting guest pages, during which we've got > existing infrastructures to retry a page fault at a later time. Allowing > the GUP to be interrupted by generic signals can make KVM related threads > to be more responsive. For examples: > > (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI, > e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be > generated to kick the vcpus out of kernel context immediately, > > (2) SIGINT: which can be used with interactive hypervisor users to stop a > virtual machine with Ctrl-C without any delays/hangs, > > (3) SIGTRAP: which grants GDB capability even during page faults that are > stuck for a long time. > > Normally hypervisor will be able to receive these signals properly, but not > if we're stuck in a GUP for a long time for whatever reason. It happens > easily with a stucked postcopy migration when e.g. a network temp failure > happens, then some vcpu threads can hang death waiting for the pages. With > the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively > enable the ability to trap these signals. > > Reviewed-by: John Hubbard > Reviewed-by: David Hildenbrand > Signed-off-by: Peter Xu If no one shouts I will queue the series for 6.2, but it would be nice to get an ACK from the mm folks. > --- > include/linux/mm.h | 1 + > mm/gup.c | 33 +++++++++++++++++++++++++++++---- > mm/hugetlb.c | 5 ++++- > 3 files changed, 34 insertions(+), 5 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 21f8b27bd9fd..488a9f4cce07 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2897,6 +2897,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, > #define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */ > #define FOLL_PIN 0x40000 /* pages must be released via unpin_user_page */ > #define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */ > +#define FOLL_INTERRUPTIBLE 0x100000 /* allow interrupts from generic signals */ > > /* > * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each > diff --git a/mm/gup.c b/mm/gup.c > index 5abdaf487460..d51e7ccaef32 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -970,8 +970,17 @@ static int faultin_page(struct vm_area_struct *vma, > fault_flags |= FAULT_FLAG_WRITE; > if (*flags & FOLL_REMOTE) > fault_flags |= FAULT_FLAG_REMOTE; > - if (locked) > + if (locked) { > fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE; > + /* > + * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set > + * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE. > + * That's because some callers may not be prepared to > + * handle early exits caused by non-fatal signals. > + */ > + if (*flags & FOLL_INTERRUPTIBLE) > + fault_flags |= FAULT_FLAG_INTERRUPTIBLE; > + } > if (*flags & FOLL_NOWAIT) > fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT; > if (*flags & FOLL_TRIED) { > @@ -1380,6 +1389,22 @@ int fixup_user_fault(struct mm_struct *mm, > } > EXPORT_SYMBOL_GPL(fixup_user_fault); > > +/* > + * GUP always responds to fatal signals. When FOLL_INTERRUPTIBLE is > + * specified, it'll also respond to generic signals. The caller of GUP > + * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption. > + */ > +static bool gup_signal_pending(unsigned int flags) > +{ > + if (fatal_signal_pending(current)) > + return true; > + > + if (!(flags & FOLL_INTERRUPTIBLE)) > + return false; > + > + return signal_pending(current); > +} > + > /* > * Please note that this function, unlike __get_user_pages will not > * return 0 for nr_pages > 0 without FOLL_NOWAIT > @@ -1461,11 +1486,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, > * Repeat on the address that fired VM_FAULT_RETRY > * with both FAULT_FLAG_ALLOW_RETRY and > * FAULT_FLAG_TRIED. Note that GUP can be interrupted > - * by fatal signals, so we need to check it before we > + * by fatal signals of even common signals, depending on > + * the caller's request. So we need to check it before we > * start trying again otherwise it can loop forever. > */ > - > - if (fatal_signal_pending(current)) { > + if (gup_signal_pending(flags)) { > if (!pages_done) > pages_done = -EINTR; > break; > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index e070b8593b37..202f3ad7f35c 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6206,9 +6206,12 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, > fault_flags |= FAULT_FLAG_WRITE; > else if (unshare) > fault_flags |= FAULT_FLAG_UNSHARE; > - if (locked) > + if (locked) { > fault_flags |= FAULT_FLAG_ALLOW_RETRY | > FAULT_FLAG_KILLABLE; > + if (flags & FOLL_INTERRUPTIBLE) > + fault_flags |= FAULT_FLAG_INTERRUPTIBLE; > + } > if (flags & FOLL_NOWAIT) > fault_flags |= FAULT_FLAG_ALLOW_RETRY | > FAULT_FLAG_RETRY_NOWAIT;