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Fri, 19 Feb 2021 05:53:07 -0800 (PST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.7\)) Subject: Re: [PATCH v2] mmp: do not use O_DIRECT when working with regular file From: Alexey Lyashkov In-Reply-To: <20210219133459.vezgrlkjpmaizvb4@work> Date: Fri, 19 Feb 2021 16:53:05 +0300 Cc: Andreas Dilger , Artem Blagodarenko , linux-ext4 , Eric Sandeen Content-Transfer-Encoding: quoted-printable Message-Id: References: <20210212093719.162065-1-lczerner@redhat.com> <20210218095146.265302-1-lczerner@redhat.com> <99A17D19-8764-4027-8B1E-E7ADBE5E2CEE@gmail.com> <20210219105713.uu2mywenytfd2e5j@work> <20210219133459.vezgrlkjpmaizvb4@work> To: Lukas Czerner X-Mailer: Apple Mail (2.3445.9.7) Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org > 19 =D1=84=D0=B5=D0=B2=D1=80. 2021 =D0=B3., =D0=B2 16:34, Lukas Czerner = =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB(=D0=B0): >=20 > On Fri, Feb 19, 2021 at 02:49:16PM +0300, Alexey Lyashkov wrote: >> Lukas, >>=20 >> because e2fsprogs have an bad assumption about IO size for the = O_DIRECT case. >> and because library uses a code like >>>>=20 >> set_block_size(1k); >> seek(fs, 1); >> read_block(); >>>>>=20 >> which caused an 1k read inside of 4k disk block size not aligned by = block size, which is prohibited and caused an error report. >>=20 >> Reference to patch. >> = https://patchwork.ozlabs.org/project/linux-ext4/patch/20201023112659.1559-= 1-artem.blagodarenko@gmail.com/ >=20 > Alright, I skimmed through your patch proposal and I am not sure I > completely understand the problem because you have not provided the = code > adding O_DIRECT support for e2image. debugfs -D =E2=80=A6 will hit same problem. >=20 > However I think that it is a reasonable assumption to make that there = is > not going to be a file system on a block device such that the fs = blocksize > is smaller than device sector size. You can't create such fs with = mke2fs > and you can't mount such file system either. >=20 This is don=E2=80=99t need to be create this FS, calling an ext2_open2 = is enough. > All that said I can now see that there is a problem in case of mke2fs > and debugfs when used with O_DIRECT (-D) on a file system image with = 1k > block size stored on a file in the host file system on the block = device > with sector size larger than 1k (...I am getting Inception flashbacks = now) if you have open a large (>256T) device with debugfs without -D you will = be see a large swap. once this FS want to consume a ~10G for bitmaps and other parts. with cached read you memory consumption increased by two. btw. you can easy replicate it with losetup which able to specify a block = size. >=20 > In fact I can confirm that indeed, both mke2fs and debugfs will fail = in > such scenario. The question is whether we care enough to support > O_DIRECT in such situations. Personally I don't care enough about = this. > However it would be nice to at least have a check (probably in > ext2fs_open2, unix_open_channel or such) and notify user about the > problem. it=E2=80=99s not a tools problem. It=E2=80=99s problem of e2fsprogs = library as ext2_open2 affected by this bug. But this is not a single function where bug lives. >=20 > Note that this conversation does not affect my patch since > ext2fs_mmp_read() does not use the unix_io infrastructure. >=20 It=E2=80=99s good to convert to use it. Alex > -Lukas >=20 >>=20 >> Alex >>=20 >>> 19 =D1=84=D0=B5=D0=B2=D1=80. 2021 =D0=B3., =D0=B2 13:57, Lukas = Czerner =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 =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 = 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 >>>>>> Reviewed-by: Eric Sandeen >>>>>=20 >>>>> Reviewed-by: Andreas Dilger >>>>>=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 >>=20 >=20