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[99.241.198.116]) by smtp.gmail.com with ESMTPSA id h8-20020a92d848000000b002da9f82c703sm2049757ilq.5.2022.06.28.12.31.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Jun 2022 12:31:39 -0700 (PDT) Date: Tue, 28 Jun 2022 15:31:37 -0400 From: Peter Xu To: John Hubbard Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , Andrew Morton , David Hildenbrand , "Dr . David Alan Gilbert" , Andrea Arcangeli , Linux MM Mailing List , Sean Christopherson Subject: Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE Message-ID: References: <20220622213656.81546-1-peterx@redhat.com> <20220622213656.81546-2-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, John, Thanks for your comments! On Mon, Jun 27, 2022 at 07:07:28PM -0700, John Hubbard wrote: [...] > > @@ -2941,6 +2941,7 @@ 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 */ > > Perhaps, s/generic/non-fatal/ ? Sure. > > diff --git a/mm/gup.c b/mm/gup.c > > index 551264407624..ad74b137d363 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -933,8 +933,17 @@ 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; > > + /* > > + * We should only grant FAULT_FLAG_INTERRUPTIBLE when we're > > + * (at least) killable. It also mostly means we're not > > + * with NOWAIT. Otherwise ignore FOLL_INTERRUPTIBLE since > > + * it won't make a lot of sense to be used alone. > > + */ > > This comment seems a little confusing due to its location. We've just > checked "locked", but the comment is talking about other constraints. > > Not sure what to suggest. Maybe move it somewhere else? I put it here to be after FAULT_FLAG_KILLABLE we just set. Only if we have "locked" will we set FAULT_FLAG_KILLABLE. That's also the key we grant "killable" attribute to this GUP. So I thought it'll be good to put here because I want to have FOLL_INTERRUPTIBLE dependent on "locked" being set. > > > + 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) { > > @@ -1322,6 +1331,22 @@ 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); > > +} > > + > > OK. > > > /* > > * Please note that this function, unlike __get_user_pages will not > > * return 0 for nr_pages > 0 without FOLL_NOWAIT > > @@ -1403,11 +1428,11 @@ 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)) { > > This is new and bold. :) Signals that an application was prepared to > handle can now cause gup to quit early. I wonder if that will break any > use cases out there (SIGPIPE...) ? Note: I introduced the new FOLL_INTERRUPTIBLE flag, so only if the caller explicitly passing in that flag could there be a functional change. IOW, no functional change intended for this single patch, not before I start to let KVM code passing over that flag. > > Generally, gup callers handle failures pretty well, so it's probably > not too bad. But I wanted to mention the idea that handled interrupts > might be a little surprising here. Yes as I mentioned anyway it'll be an opt-in flag, so by default we don't need to worry at all, IMHO, because it should really work exactly like before, otherwise I had a bug somewhere else.. :) Thanks, -- Peter Xu