Re: [PATCH RESEND V12 1/8] fs: Generic function to convert iocb to rw flags

[Alessio Balsini <balsini@android.com>]

On Wed, Mar 24, 2021 at 03:43:12PM +0800, Rokudo Yan wrote:
> On 1/26/21 12:46 AM, Alessio Balsini wrote:
> > On Mon, Jan 25, 2021 at 03:30:50PM +0000, Alessio Balsini wrote:
> > > OverlayFS implements its own function to translate iocb flags into rw
> > > flags, so that they can be passed into another vfs call.
> > > With commit ce71bfea207b4 ("fs: align IOCB_* flags with RWF_* flags")
> > > Jens created a 1:1 matching between the iocb flags and rw flags,
> > > simplifying the conversion.
> > > 
> > > Reduce the OverlayFS code by making the flag conversion function generic
> > > and reusable.
> > > 
> > > Signed-off-by: Alessio Balsini <balsini@android.com>
> > > ---
> > >   fs/overlayfs/file.c | 23 +++++------------------
> > >   include/linux/fs.h  |  5 +++++
> > >   2 files changed, 10 insertions(+), 18 deletions(-)
> > > 
> > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > > index bd9dd38347ae..56be2ffc5a14 100644
> > > --- a/fs/overlayfs/file.c
> > > +++ b/fs/overlayfs/file.c
> > > @@ -15,6 +15,8 @@
> > >   #include <linux/fs.h>
> > >   #include "overlayfs.h"
> > > +#define OVL_IOCB_MASK (IOCB_DSYNC | IOCB_HIPRI | IOCB_NOWAIT | IOCB_SYNC)
> > > +
> > >   struct ovl_aio_req {
> > >   	struct kiocb iocb;
> > >   	struct kiocb *orig_iocb;
> > > @@ -236,22 +238,6 @@ static void ovl_file_accessed(struct file *file)
> > >   	touch_atime(&file->f_path);
> > >   }
> > > -static rwf_t ovl_iocb_to_rwf(int ifl)
> > > -{
> > > -	rwf_t flags = 0;
> > > -
> > > -	if (ifl & IOCB_NOWAIT)
> > > -		flags |= RWF_NOWAIT;
> > > -	if (ifl & IOCB_HIPRI)
> > > -		flags |= RWF_HIPRI;
> > > -	if (ifl & IOCB_DSYNC)
> > > -		flags |= RWF_DSYNC;
> > > -	if (ifl & IOCB_SYNC)
> > > -		flags |= RWF_SYNC;
> > > -
> > > -	return flags;
> > > -}
> > > -
> > >   static void ovl_aio_cleanup_handler(struct ovl_aio_req *aio_req)
> > >   {
> > >   	struct kiocb *iocb = &aio_req->iocb;
> > > @@ -299,7 +285,8 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
> > >   	old_cred = ovl_override_creds(file_inode(file)->i_sb);
> > >   	if (is_sync_kiocb(iocb)) {
> > >   		ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
> > > -				    ovl_iocb_to_rwf(iocb->ki_flags));
> > > +				    iocb_to_rw_flags(iocb->ki_flags,
> > > +						     OVL_IOCB_MASK));
> > >   	} else {
> > >   		struct ovl_aio_req *aio_req;
> > > @@ -356,7 +343,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> > >   	if (is_sync_kiocb(iocb)) {
> > >   		file_start_write(real.file);
> > >   		ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
> > > -				     ovl_iocb_to_rwf(ifl));
> > > +				     iocb_to_rw_flags(ifl, OVL_IOCB_MASK));
> > >   		file_end_write(real.file);
> > >   		/* Update size */
> > >   		ovl_copyattr(ovl_inode_real(inode), inode);
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index fd47deea7c17..647c35423545 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -3275,6 +3275,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> > >   	return 0;
> > >   }
> > > +static inline rwf_t iocb_to_rw_flags(int ifl, int iocb_mask)
> > > +{
> > > +	return ifl & iocb_mask;
> > > +}
> > > +
> > >   static inline ino_t parent_ino(struct dentry *dentry)
> > >   {
> > >   	ino_t res;
> > > -- 
> > > 2.30.0.280.ga3ce27912f-goog
> > > 
> > 
> > For some reason lkml.org and lore.kernel.org are not showing this change
> > as part of the thread.
> > Let's see if replying to the email fixes the indexing.
> > 
> > Regards,
> > Alessio
> > 
> 
> Hi, Alessio
> 
> This change imply IOCB_* and RWF_* flags are properly aligned, which is not
> true for kernel version 5.4/4.19/4.14. As the patch ("fs: align IOCB_* flags
> with RWF_* flags") is not back-ported to these stable kernel branches. The
> issue was found when applying these patches
> to kernel-5.4(files open with passthrough enabled can't do append write). I
> think the issue exists in AOSP common kernel too.
> Could you please fix this?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce71bfea207b4d7c21d36f24ec37618ffcea1da8
> 
> https://android-review.googlesource.com/c/kernel/common/+/1556243
> 
> Thanks
> yanwu

Hi yanwu,

Correct, this change depends on commit ce71bfea207b ("fs: align IOCB_*
flags with RWF_* flags"), and this dependency is satisfied upstream.
Being FUSE passthrough a new feature and not a bugfix, I'm not planning
to do any backporting to LTS kernels (and GregKH won't probably accept
it).

Android is a different story (and slightly out of topic here).
We are looking forward to have FUSE passthrough enabled on Android as
most of the user data is handled by FUSE. We liked the performance
improvements and non-intrusiveness of the change both for the kernel and
for userspace, so we started supporting this in android12-5.4+ kernel
branches. We are not planning to maintain the feature to older kernels
though (we can't add features to already released), and this is why FUSE
passthrough is not merged there.
To answer your question, in AOSP the officially supported kernels
already have the flags alignment change merged, and a not supported
backporting to older kernels (i.e., 4.14 and 4.19) is already available:

https://android-review.googlesource.com/q/%2522BACKPORT:+fs:+align+IOCB_*+flags+with+RWF_*+flags%2522+-status:abandoned

Thanks,
Alessio