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Date: Wed, 6 Mar 2024 12:16:00 +0100
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Subject: Re: [PATCH v1] fs/fuse: Fix missing FOLL_PIN for direct-io
To: Miklos Szeredi <miklos@szeredi.hu>, Lei Huang <lei.huang@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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From: Bernd Schubert <bernd.schubert@fastmail.fm>
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On 3/6/24 11:01, Miklos Szeredi wrote:
> 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.

Hi Miklos,

do you have an idea if this needs to be back ported and to which kernel
version?
I had tried to reproduce data corruption with 4.18 - Lei wrote that he
could see issues with older kernels as well, but I never managed to
trigger anything on 4.18-RHEL. Typically I use ql-fstest
(https://github.com/bsbernd/ql-fstest) and even added random DIO as an
option - nothing report with weeks of run time. I could try again with
more recent kernels that have folios.

Thanks,
Bernd