Messages in this thread | | | Date | Tue, 16 Sep 2014 14:01:15 -0700 | Subject | Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem | From | Andres Lagar-Cavilla <> |
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On Tue, Sep 16, 2014 at 1:51 PM, Radim Krčmář <rkrcmar@redhat.com> wrote: > 2014-09-15 13:11-0700, Andres Lagar-Cavilla: >> +int kvm_get_user_page_retry(struct task_struct *tsk, struct mm_struct *mm, > > The suffix '_retry' is not best suited for this. > On first reading, I imagined we will be retrying something from before, > possibly calling it in a loop, but we are actually doing the first and > last try in one call.
We are doing ... the second and third in most scenarios. async_pf did the first with _NOWAIT. We call this from the async pf retrier, or if async pf couldn't be notified to the guest.
> > Hard to find something that conveys our lock-dropping mechanic, > '_polite' is my best candidate at the moment.
I'm at a loss towards finding a better name than '_retry'.
> >> + int flags = FOLL_TOUCH | FOLL_HWPOISON | > > (FOLL_HWPOISON wasn't used before, but it's harmless.)
Ok. Wasn't 100% sure TBH.
> > 2014-09-16 15:51+0200, Paolo Bonzini: >> Il 15/09/2014 22:11, Andres Lagar-Cavilla ha scritto: >> > @@ -1177,9 +1210,15 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault, >> > npages = get_user_page_nowait(current, current->mm, >> > addr, write_fault, page); >> > up_read(¤t->mm->mmap_sem); >> > - } else >> > - npages = get_user_pages_fast(addr, 1, write_fault, >> > - page); >> > + } else { >> > + /* >> > + * By now we have tried gup_fast, and possible async_pf, and we > ^ > (If we really tried get_user_pages_fast, we wouldn't be here, so I'd > prepend two underscores here as well.)
Yes, async pf tries and fails to do fast, and then we fallback to slow, and so on.
> >> > + * are certainly not atomic. Time to retry the gup, allowing >> > + * mmap semaphore to be relinquished in the case of IO. >> > + */ >> > + npages = kvm_get_user_page_retry(current, current->mm, addr, >> > + write_fault, page); >> >> This is a separate logical change. Was this: >> >> down_read(&mm->mmap_sem); >> npages = get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL); >> up_read(&mm->mmap_sem); >> >> the intention rather than get_user_pages_fast? > > I believe so as well. > > (Looking at get_user_pages_fast and __get_user_pages_fast made my > abstraction detector very sad.)
It's clunky, but a separate battle.
> >> I think a first patch should introduce kvm_get_user_page_retry ("Retry a >> fault after a gup with FOLL_NOWAIT.") and the second would add >> FOLL_TRIED ("This properly relinquishes mmap semaphore if the >> filemap/swap has to wait on page lock (and retries the gup to completion >> after that"). > > Not sure if that would help to understand the goal ... > >> Apart from this, the patch looks good. The mm/ parts are minimal, so I >> think it's best to merge it through the KVM tree with someone's Acked-by. > > I would prefer to have the last hunk in a separate patch, but still, > > Acked-by: Radim Krčmář <rkrcmar@redhat.com> Awesome, thanks much.
I'll recut with the VM_BUG_ON from Paolo and your Ack. LMK if anything else from this email should go into the recut.
Andres
-- Andres Lagar-Cavilla | Google Kernel Team | andreslc@google.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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