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From: Miklos Szeredi <miklos@szeredi.hu>
Date: Wed, 6 Mar 2024 11:01:17 +0100
Message-ID: <CAJfpegtX_XAHhHS4XN1-=cOHy0ZUSxuA_OQO5tdujLVJdE1EdQ@mail.gmail.com>
Subject: Re: [PATCH v1] fs/fuse: Fix missing FOLL_PIN for direct-io
To: Lei Huang <lei.huang@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Tue, 29 Aug 2023 at 20:37, Lei Huang <lei.huang@linux.intel.com> wrote:
>
> Our user space filesystem relies on fuse to provide POSIX interface.
> In our test, a known string is written into a file and the content
> is read back later to verify correct data returned. We observed wrong
> data returned in read buffer in rare cases although correct data are
> stored in our filesystem.
>
> Fuse kernel module calls iov_iter_get_pages2() to get the physical
> pages of the user-space read buffer passed in read(). The pages are
> not pinned to avoid page migration. When page migration occurs, the
> consequence are two-folds.
>
> 1) Applications do not receive correct data in read buffer.
> 2) fuse kernel writes data into a wrong place.
>
> Using iov_iter_extract_pages() to pin pages fixes the issue in our
> test.
>
> An auxiliary variable "struct page **pt_pages" is used in the patch
> to prepare the 2nd parameter for iov_iter_extract_pages() since
> iov_iter_get_pages2() uses a different type for the 2nd parameter.
>
> Signed-off-by: Lei Huang <lei.huang@linux.intel.com>

Applied, with a modification to only unpin if
iov_iter_extract_will_pin() returns true.

Thanks,
Miklos