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Date:   Tue, 6 Feb 2018 15:19:15 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        ross.zwisler@...ux.intel.com, linux-ext4@...r.kernel.org,
        dan.j.williams@...el.com, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 2 2/2] xfs: fix rt_dev usage for DAX

On Tue, Feb 06, 2018 at 03:32:00PM -0700, Dave Jiang wrote:
> On 02/01/2018 05:43 PM, Darrick J. Wong wrote:
> > On Fri, Feb 02, 2018 at 10:44:13AM +1100, Dave Chinner wrote:
> >> On Thu, Feb 01, 2018 at 01:33:05PM -0700, Dave Jiang wrote:
> >>> When using realtime device (rtdev) with xfs where the data device is not
> >>> DAX capable, two issues arise. One is when data device is not DAX but the
> >>> realtime device is DAX capable, we currently disable DAX.
> >>> After passing this check, we are also not marking the inode as DAX capable.
> >>> This change will allow DAX enabled if the data device or the realtime
> >>> device is DAX capable. S_DAX will be marked for the inode if the file is
> >>> residing on a DAX capable device. This will prevent the case of rtdev is not
> >>> DAX and data device is DAX to create realtime files.
> >>
> >> I'm confused by this description. I'm not sure what is broken, nor
> >> what you are trying to fix.
> >>
> >> I think what you want to do is enable DAX on RT devices separately
> >> to the data device and vice versa?
> >>
> >> i.e. is this what you are trying to acheive?
> >>
> >> datadev dax	rtdev dax		DAX enabled on
> >> -----------     ---------		--------------
> >> no		no			neither
> >> yes		no			datadev
> >> no		yes			rtdev
> >> yes		yes			both
> >>
> >>
> >>>
> >>> Signed-off-by: Dave Jiang <dave.jiang@...el.com>
> >>> Reported-by: Darrick Wong <darrick.wong@...cle.com>
> >>> ---
> >>>  fs/xfs/xfs_iops.c  |    3 ++-
> >>>  fs/xfs/xfs_super.c |    9 ++++++++-
> >>>  2 files changed, 10 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> >>> index 56475fcd76f2..ab352c325301 100644
> >>> --- a/fs/xfs/xfs_iops.c
> >>> +++ b/fs/xfs/xfs_iops.c
> >>> @@ -1204,7 +1204,8 @@ xfs_diflags_to_iflags(
> >>>  	    ip->i_mount->m_sb.sb_blocksize == PAGE_SIZE &&
> >>>  	    !xfs_is_reflink_inode(ip) &&
> >>>  	    (ip->i_mount->m_flags & XFS_MOUNT_DAX ||
> >>> -	     ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
> >>> +	     ip->i_d.di_flags2 & XFS_DIFLAG2_DAX) &&
> >>> +	    blk_queue_dax(bdev_get_queue(inode->i_sb->s_bdev)))
> >>
> >> This does not discriminate between the rtdev or the data dev. This
> >> needs to call xfs_find_bdev_for_inode() to get the right device
> >> for the inode config.
> >>
> >> Further, if we add or remove the RT flag to the inode at a later
> >> point in time (e.g. via ioctl) we also need to re-evaluate the S_DAX
> >> flag at that point in time.
> > 
> > Ah, right, I'd missed that subtlety in my earlier replies.  Ok, add
> > another patch to this series to reevaluate S_DAX when we change the RT
> > flag.
> > 
> >> Which brings me to the real problem here: dynamically changing the
> >> S_DAX flag is racy, dangerous and broken. It's not clear that this
> >> should be allowed at all as the inode may have already been mmap()d
> >> by the time the ioctl is called to set/clear the rt file state.
>
> > Agreed that this is a mess.  Either this needs to get fixed in the dax
> > code, or we need to decide that we're not going to support reconfiguring
> > the dax flag at all, except possibly for empty files (similar to how we
> > restrict changes to the rt flag).
> 
> Does this mean we should add a check in xfs_ioctl_setattr_xflags() to
> reject removing of realtime flag if S_DAX is set on the inode until the
> dynamic change issue is sorted out?

Well, I /was/ tiptoeing around this while trying to take care of the
other 4.16 stuff, but now that I'm done with merge window stuff I'll let
loose. :)

The last time I paid much attention to DAX was the thread "re-enable XFS
per-inode DAX"[1] last September.  Motivating me to merge anything else
into DAX involves convincing me that we (mm, fs, dax developers) have
some kind of agreement about what we want the user-visible interfaces to
DAX to look like.  Namely:

0. On what level do we allow users / administrators to control usage of
the dax paths?  Can the hardware convey enough detail to the kernel that
the kernel can make a reasonable decision on its own whether buffered or
dax io make more sense?  If so, can we please just have that?  If not,
why?

1. If we want to let users override whatever decision the kernel makes,
how should we do this?  One mount option that applies to everything,
like ext4?  Inheritable inode flags, like xfs?  Do we have one to force
it on even if the kernel doesn't want to?  Do we have another to force
it off even if the kernel wants to?  Do we even want to go down this
path?  Can we get away with making the answer to Q0 "yes" and then see
if anyone actually complains about not having fine-grained control?

2. Under what conditions can we support dynamic changing of S_DAX on
inodes at runtime?  Will this switching work at any time?  Only for
files that are open but not mmap'd?  Only for files that are empty?

3. The MAP_SYNC support that was merged into 4.15 -- is this sufficient
to allow this fsyncless clflush business that everyone seems to want?

4. Can someone please fix the XFS iomap_begin function to handle CoW
properly?  I think it's a simple matter of allocate blocks, memcpy, and
remap, though I don't know how to do that. ;)

5. Do we test any of this stuff?

The thread from last September left off with promises to go define what
interface and behaviors we are providing to userspace, but afaict none
of that ever happened?  If we don't resolve these questions before LSF
then I think what's needed is to lock everyone in a room to hash all
this out. :P

--D

PS: My personal inclination is {yes, get rid of all that until someone
complains, i think so but haven't tested it, ???, i sure hope so}.

[1] https://marc.info/?l=linux-xfs&m=150638135225793&w=2

> 
> > 
> >> IOWs, right now we cannot support mixed DAX mode filesystems because
> >> the generic DAX code does not support dynamic changing of the DAX
> >> flag on an inode and so checking the block device state here is
> >> irrelevant....
> >>
> >>>  		inode->i_flags |= S_DAX;
> >>>  }
> >>>  
> >>> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> >>> index e8a687232614..5ac478924dce 100644
> >>> --- a/fs/xfs/xfs_super.c
> >>> +++ b/fs/xfs/xfs_super.c
> >>> @@ -1649,11 +1649,18 @@ xfs_fs_fill_super(
> >>>  		sb->s_flags |= SB_I_VERSION;
> >>>  
> >>>  	if (mp->m_flags & XFS_MOUNT_DAX) {
> >>> +		bool rtdev_is_dax = false;
> >>> +
> >>>  		xfs_warn(mp,
> >>>  		"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
> >>>  
> >>> +		if (mp->m_rtdev_targp->bt_daxdev)
> >>> +			if (bdev_dax_supported(mp->m_rtdev_targp->bt_bdev,
> >>> +					      sb->s_blocksize) == 0)
> >>> +				rtdev_is_dax = true;
> >>
> >> .... as this code here needs to turn off DAX here if any device
> >> in the filesystem doesn't support DAX....
> > 
> > I think it'd be useful to be able to have a pmem rt device even if the
> > data device doesn't support it.  Or rather, I have a few clients who
> > have expressed interest in this sort of configuration.
> > 
> > --D
> > 
> >>
> >>
> >> FWIW, the logic in the code is terrible (not your fault, Dave).
> >> The logic reads
> >>
> >> 	if (NOT bdev_dax_supported(rtdev)) then
> >> 		rtdev supports DAX
> >>
> >> That also needs fixing - we're checking something that has a boolean
> >> return state (yes or no) and so it should define them in a way that
> >> makes the caller logic read cleanly....
> >>
> >> Cheers,
> >>
> >> Dave.
> >> -- 
> >> Dave Chinner
> >> david@...morbit.com
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
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