linux-ext4 - Re: [PATCH v4 12/13] ext4: Stop providing .writepage hook

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Message-ID: <CAOzgRdbkno+k1_vFfH9XVPcWxG7YCQRUWC2sX6kMSE3_gLODfA@mail.gmail.com>
Date:   Wed, 10 May 2023 13:17:51 +0800
From:   youling 257 <youling257@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Eric Biggers <ebiggers@...nel.org>, jack@...e.cz,
        hch@...radead.org, hch@....de, linux-ext4@...r.kernel.org,
        ritesh.list@...il.com, keescook@...omium.org
Subject: Re: [PATCH v4 12/13] ext4: Stop providing .writepage hook

It isn't android storage emulated esdfs or sdcardfs problem, i test
mount bind /data/media on /storage/emulated, can reproduce my problem.
Let me say clear my problem, "ext4: Stop providing .writepage hook"
cause android gallery app unable read pictures thumbnails.

I test linux kernel 6.3 Revert "ext4: Stop providing .writepage hook",
android gallery can read pictures thumbnails,

this is mount bind /data/media on /storage/emulated
/dev/nvme0n1p1 on /storage/emulated type ext4 (rw,seclabel,noatime)
/dev/nvme0n1p1 on /mnt/runtime/default/emulated type ext4 (rw,seclabel,noatime)
/dev/nvme0n1p1 on /mnt/runtime/read/emulated type ext4 (rw,seclabel,noatime)
/dev/nvme0n1p1 on /mnt/runtime/write/emulated type ext4 (rw,seclabel,noatime)
/dev/nvme0n1p1 on /mnt/runtime/full/emulated type ext4 (rw,seclabel,noatime)

this is esdfs,
/data/media on /mnt/runtime/default/emulated type esdfs
(rw,nosuid,nodev,noexec,noatime,lower=1023:1023:664:775,upper=0:1015:771:771,derive=multi,noconfine,derive_gid,default_normal,unshared_obb)
/data/media on /storage/emulated type esdfs
(rw,nosuid,nodev,noexec,noatime,lower=1023:1023:664:775,upper=0:1015:771:771,derive=multi,noconfine,derive_gid,default_normal,unshared_obb)
/data/media on /mnt/runtime/read/emulated type esdfs
(rw,nosuid,nodev,noexec,noatime,lower=1023:1023:664:775,upper=0:9997:750:750,derive=multi,noconfine,derive_gid,default_normal,unshared_obb)
/data/media on /mnt/runtime/write/emulated type esdfs
(rw,nosuid,nodev,noexec,noatime,lower=1023:1023:664:775,upper=0:9997:770:770,derive=multi,noconfine,derive_gid,default_normal,unshared_obb)
/data/media on /mnt/runtime/full/emulated type esdfs
(rw,nosuid,nodev,noexec,noatime,lower=1023:1023:664:775,upper=0:9997:770:770,derive=multi,noconfine,derive_gid,default_normal,unshared_obb)

if i do mount bind data/media on storage/emulated, take a screenshot,
will create /data/media/0/Pictures/Screenshots/Screenshot_20230510-130020.png
file,
chmod -R 0777 /data/media/0/Pictures/Screenshots,
on linux 6.3 Revert "ext4: Stop providing .writepage hook", android
gallery app can read
/storage/emulated/0/Pictures/Screenshots/Screenshot_20230510-130020.png
thumbnail,
linux 6.4 removed .writepage hook, android gallery unable read thumbnail.


2023-05-10 2:36 GMT+08:00, Theodore Ts'o <tytso@....edu>:
> On Mon, May 08, 2023 at 10:02:27PM -0700, Eric Biggers wrote:
>> On Tue, May 09, 2023 at 01:51:08AM +0800, youling257 wrote:
>> > I using linux mainline kernel on android.
>> > https://github.com/youling257/android-mainline/commits/6.4
>> > https://github.com/youling257/android-mainline/commits/6.3
>> > "ext4: Stop providing .writepage hook" cause some android app unable to
>> > read storage/emulated/0 files, i need to say android esdfs file system
>> > storage/emulated is ext4 data/media bind mount.
>> > I want to ask, why android storage/emulated need .writepage hook?
>>
>> "esdfs" doesn't exist upstream, so linux-ext4 can't provide support for
>> it.
>>
>> Also, it doesn't exist in the Android Common Kernels either, so the
>> Android team
>> cannot help you either.
>
> The problem with esdfs is that it's based on the old stackable file
> system paradigm which is filled with races and is inherently
> unreliable (just for fun, try running fsstress on the upper and lower
> file systems of a stackable file system simultaneously, and watch the
> kernel crash and burn).  For that reason, some number of us have been
> working for a while to eliminate the need for stacking file systems,
> such as sdcardfs. esdfs, etc. from the Android kernel.
>
> The other thing I would add is that upstream has been working[1] on
> getting rid of writepage function.  So out-of-tree file systems are
> going to need to adapt --- or die.
>
> [1] https://lore.kernel.org/all/20221202102644.770505-1-hch@lst.de/
>
> It looks like esdfs is coming from the Chromium kernel?  The latest
> Chromium kernel I can find is 5.15 based, and it has esdfs in it.  I'm
> sad to see that esdfs hasn't been removed from the Chromium kernel
> yet, and replaced with something more stable and reliable, but maybe
> we can find someone who is more familiar with the Chromium kernel to
> comment.
>
> Cheers,
>
> 						- Ted
>