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Subject: Re: [PATCH v2] mmp: do not use O_DIRECT when working with regular
 file
From:   Alexey Lyashkov <alexey.lyashkov@gmail.com>
In-Reply-To: <20210219105713.uu2mywenytfd2e5j@work>
Date:   Fri, 19 Feb 2021 14:49:16 +0300
Cc:     Andreas Dilger <adilger@dilger.ca>,
        Artem Blagodarenko <artem.blagodarenko@gmail.com>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        Eric Sandeen <sandeen@redhat.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <E16FB371-5DFC-4A10-A9E2-36541FCF7D30@gmail.com>
References: <20210212093719.162065-1-lczerner@redhat.com>
 <20210218095146.265302-1-lczerner@redhat.com>
 <BF8274AF-A9C6-40F4-8B99-FEBA82878C36@dilger.ca>
 <99A17D19-8764-4027-8B1E-E7ADBE5E2CEE@gmail.com>
 <20210219105713.uu2mywenytfd2e5j@work>
To:     Lukas Czerner <lczerner@redhat.com>
Precedence: bulk

Lukas,

because e2fsprogs have an bad assumption about IO size for the O_DIRECT =
case.
and because library uses a code like
>>
set_block_size(1k);
seek(fs, 1);
read_block();
>>>
which caused an 1k read inside of 4k disk block size not aligned by =
block size, which is prohibited and caused an error report.

Reference to patch.
=
https://patchwork.ozlabs.org/project/linux-ext4/patch/20201023112659.1559-=
1-artem.blagodarenko@gmail.com/

Alex

> 19 =D1=84=D0=B5=D0=B2=D1=80. 2021 =D0=B3., =D0=B2 13:57, Lukas Czerner =
<lczerner@redhat.com> =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB(=D0=B0):
>=20
> On Fri, Feb 19, 2021 at 01:08:17PM +0300, Alexey Lyashkov wrote:
>> Andreas,
>>=20
>> What about to disable a O_DIRECT global on any block devices in the =
e2fsprogs library as this don=E2=80=99t work on 4k disk drives at all ?
>> Instead of fixing an O_DIRECT access with patches sends early.
>=20
> Why would it not work at all ? This is a fix for a specific problem =
and
> I am not currently aware of ony other problems e2fsprogs should have
> with 4k sector size drives. Do you have a specific problem in mind ?
>=20
> Thanks!
> -Lukas
>=20
>>=20
>>=20
>> Alex
>>=20
>>> 19 =D1=84=D0=B5=D0=B2=D1=80. 2021 =D0=B3., =D0=B2 1:20, Andreas =
Dilger <adilger@dilger.ca> =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB(=D0=B0=
):
>>>=20
>>> On Feb 18, 2021, at 2:51 AM, Lukas Czerner <lczerner@redhat.com> =
wrote:
>>>>=20
>>>> Currently the mmp block is read using O_DIRECT to avoid any caching =
that
>>>> may be done by the VM. However when working with regular files this
>>>> creates alignment issues when the device of the host file system =
has
>>>> sector size larger than the blocksize of the file system in the =
file
>>>> we're working with.
>>>>=20
>>>> This can be reproduced with t_mmp_fail test when run on the device =
with
>>>> 4k sector size because the mke2fs fails when trying to read the mmp
>>>> block.
>>>>=20
>>>> Fix it by disabling O_DIRECT when working with regular files. I =
don't
>>>> think there is any risk of doing so since the file system layer, =
unlike
>>>> shared block device, should guarantee cache consistency.
>>>>=20
>>>> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
>>>> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
>>>=20
>>> Reviewed-by: Andreas Dilger <adilger@dilger.ca>
>>>=20
>>>> ---
>>>> v2: Fix comment - it avoids problems when the sector size is larger =
not
>>>>  smaller than blocksize
>>>>=20
>>>> lib/ext2fs/mmp.c | 22 +++++++++++-----------
>>>> 1 file changed, 11 insertions(+), 11 deletions(-)
>>>>=20
>>>> diff --git a/lib/ext2fs/mmp.c b/lib/ext2fs/mmp.c
>>>> index c21ae272..cca2873b 100644
>>>> --- a/lib/ext2fs/mmp.c
>>>> +++ b/lib/ext2fs/mmp.c
>>>> @@ -57,21 +57,21 @@ errcode_t ext2fs_mmp_read(ext2_filsys fs, =
blk64_t mmp_blk, void *buf)
>>>> 	 * regardless of how the io_manager is doing reads, to avoid =
caching of
>>>> 	 * the MMP block by the io_manager or the VM.  It needs to be =
fresh. */
>>>> 	if (fs->mmp_fd <=3D 0) {
>>>> +		struct stat st;
>>>> 		int flags =3D O_RDWR | O_DIRECT;
>>>>=20
>>>> -retry:
>>>> +		/*
>>>> +		 * There is no reason for using O_DIRECT if we're =
working with
>>>> +		 * regular file. Disabling it also avoids problems with
>>>> +		 * alignment when the device of the host file system has =
sector
>>>> +		 * size larger than blocksize of the fs we're working =
with.
>>>> +		 */
>>>> +		if (stat(fs->device_name, &st) =3D=3D 0 &&
>>>> +		    S_ISREG(st.st_mode))
>>>> +			flags &=3D ~O_DIRECT;
>>>> +
>>>> 		fs->mmp_fd =3D open(fs->device_name, flags);
>>>> 		if (fs->mmp_fd < 0) {
>>>> -			struct stat st;
>>>> -
>>>> -			/* Avoid O_DIRECT for filesystem image files if =
open
>>>> -			 * fails, since it breaks when running on tmpfs. =
*/
>>>> -			if (errno =3D=3D EINVAL && (flags & O_DIRECT) &&
>>>> -			    stat(fs->device_name, &st) =3D=3D 0 &&
>>>> -			    S_ISREG(st.st_mode)) {
>>>> -				flags &=3D ~O_DIRECT;
>>>> -				goto retry;
>>>> -			}
>>>> 			retval =3D EXT2_ET_MMP_OPEN_DIRECT;
>>>> 			goto out;
>>>> 		}
>>>> --
>>>> 2.26.2
>>>>=20
>>>=20
>>>=20
>>> Cheers, Andreas
>>>=20
>>>=20
>>>=20
>>>=20
>>>=20
>>=20
>=20