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Message-ID: <CAJu=L58z-=_KkZXpEiPjDUup8GpH7079HH39csmvgUxGkvXy0A@mail.gmail.com> Date: Tue, 16 Sep 2014 11:42:17 -0700 From: Andres Lagar-Cavilla <andreslc@...gle.com> To: Paolo Bonzini <pbonzini@...hat.com> Cc: Gleb Natapov <gleb@...hat.com>, Rik van Riel <riel@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Mel Gorman <mgorman@...e.de>, Andy Lutomirski <luto@...capital.net>, Andrew Morton <akpm@...ux-foundation.org>, Andrea Arcangeli <aarcange@...hat.com>, Sasha Levin <sasha.levin@...cle.com>, Jianyu Zhan <nasa4836@...il.com>, Paul Cassella <cassella@...y.com>, Hugh Dickins <hughd@...gle.com>, Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini <pbonzini@...hat.com> wrote: > Il 16/09/2014 18:52, Andres Lagar-Cavilla ha scritto: >> 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 meant the intention of the original author, not yours. Yes, in all likelihood. I hope! > >> By that point in the call chain I felt comfortable dropping the _fast. >> All paths that get there have already tried _fast (and some have tried >> _NOWAIT). > > Yes, understood. > >> 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"). >> >> That's not what FOLL_TRIED does. The relinquishing of mmap semaphore is >> done by this patch minus the FOLL_TRIED bits. FOLL_TRIED will let the >> fault handler (e.g. filemap) know that we've been there and waited on >> the IO already, so in the common case we won't need to redo the IO. > > Yes, that's not what FOLL_TRIED does. But it's the difference between > get_user_pages and kvm_get_user_page_retry, right? Unfortunately get_user_pages does not expose the param (int *nonblocking) that __gup will use to set FAULT_FLAG_ALLOW_RETRY. So that's one difference. The second difference is that kvm_gup_retry will call two times if necessary (the second without _RETRY but with _TRIED). Thanks Andres > > Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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