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Subject: Re: [Qemu-devel] [PATCH 01/17] mm: gup: add FOLL_TRIED
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Robert Love <rlove@google.com>, Dave Hansen <dave@sr71.net>, Jan Kara <jack@suse.cz>, KVM list <kvm@vger.kernel.org>, Neil Brown <neilb@suse.de>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, linux-mm <linux-mm@kvack.org>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Michel Lespinasse <walken@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Taras Glek <tglek@mozilla.com>, Andrew Jones <drjones@redhat.com>, Juan Quintela <quintela@redhat.com>, Hugh Dickins <hughd@google.com>, Isaku Yamahata <yamahata@valinux.co.jp>, Mel Gorman <mgorman@suse.de>, Sasha Levin <sasha.levin@oracle.com>, Android Kernel Team <kernel-team@android.com>, "\\Dr. David Alan Gilbert" <dgilbert@redhat.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Andres Lagar-Cavilla <andreslc@google.com>, Christopher Covington <cov@codeaurora.org>, Anthony Liguori <anthony@codemonkey.ws>, Keith Packard <keithp@keithp.com>, Wenchao Xia <wenchaoqemu@gmail.com>, Linux API <linux-api@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, Minchan Kim <minchan@kernel.org>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Mike Hommey <mh@glandium.org>, Andrew Morton <akpm@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

> This needs more explanation than that one-liner comment. Make the
> commit message explain why the new FOLL_TRIED flag exists.

This patch actually is extracted from a 3.18 commit in the KVM tree,
https://git.kernel.org/cgit/virt/kvm/kvm.git/commit/?h=next&id=234b239b.

Here is how that patch uses the flag:

    /*
     * The previous call has now waited on the IO. Now we can
     * retry and complete. Pass TRIED to ensure we do not re
     * schedule async IO (see e.g. filemap_fault).
     */
    down_read(&mm->mmap_sem);
    npages = __get_user_pages(tsk, mm, addr, 1, flags | FOLL_TRIED,
                              pagep, NULL, NULL);