Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

[John Hubbard]

On 6/5/19 6:45 PM, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ... V1,000,000   ;-)
> 
> Pre-requisites:
> 	John Hubbard's put_user_pages() patch series.[1]
> 	Jan Kara's ext4_break_layouts() fixes[2]
> 
> Based on the feedback from LSFmm and the LWN article which resulted.  I've
> decided to take a slightly different tack on this problem.
> 
> The real issue is that there is no use case for a user to have RDMA pinn'ed
> memory which is then truncated.  So really any solution we present which:
> 
> A) Prevents file system corruption or data leaks
> ...and...
> B) Informs the user that they did something wrong
> 
> Should be an acceptable solution.
> 
> Because this is slightly new behavior.  And because this is gonig to be
> specific to DAX (because of the lack of a page cache) we have made the user
> "opt in" to this behavior.
> 
> The following patches implement the following solution.
> 
> 1) The user has to opt in to allowing GUP pins on a file with a layout lease
>    (now made visible).
> 2) GUP will fail (EPERM) if a layout lease is not taken
> 3) Any truncate or hole punch operation on a GUP'ed DAX page will fail.
> 4) The user has the option of holding the layout lease to receive a SIGIO for
>    notification to the original thread that another thread has tried to delete
>    their data.  Furthermore this indicates that if the user needs to GUP the
>    file again they will need to retake the Layout lease before doing so.
> 
> 
> NOTE: If the user releases the layout lease or if it has been broken by another
> operation further GUP operations on the file will fail without re-taking the
> lease.  This means that if a user would like to register pieces of a file and
> continue to register other pieces later they would be advised to keep the
> layout lease, get a SIGIO notification, and retake the lease.
> 
> NOTE2: Truncation of pages which are not actively pinned will succeed.  Similar
> to accessing an mmap to this area GUP pins of that memory may fail.
> 

Hi Ira,

Wow, great to see this. This looks like basically the right behavior, IMHO.

1. We'll need man page additions, to explain it. In fact, even after a quick first
pass through, I'm vague on two points:

a) I'm not sure how this actually provides "opt-in to new behavior", because I 
don't see any CONFIG_* or boot time choices, and it looks like the new behavior 
just is there. That is, if user space doesn't set F_LAYOUT on a range, 
GUP FOLL_LONGTERM will now fail, which is new behavior. (Did I get that right?)

b) Truncate and hole punch behavior, with and without user space having a SIGIO
handler. (I'm sure this is obvious after another look through, but it might go
nicely in a man page.)

2. It *seems* like ext4, xfs are taken care of here, not just for the DAX case,
but for general RDMA on them? Or is there more that must be done?

3. Christophe Hellwig's unified gup patchset wreaks havoc in gup.c, and will
conflict violently, as I'm sure you noticed. :)


thanks,
-- 
John Hubbard
NVIDIA

> 
> A general overview follows for background.
> 
> It should be noted that one solution for this problem is to use RDMA's On
> Demand Paging (ODP).  There are 2 big reasons this may not work.
> 
> 	1) The hardware being used for RDMA may not support ODP
> 	2) ODP may be detrimental to the over all network (cluster or cloud)
> 	   performance
> 
> Therefore, in order to support RDMA to File system pages without On Demand
> Paging (ODP) a number of things need to be done.
> 
> 1) GUP "longterm" users need to inform the other subsystems that they have
>    taken a pin on a page which may remain pinned for a very "long time".[3]
> 
> 2) Any page which is "controlled" by a file system needs to have special
>    handling.  The details of the handling depends on if the page is page cache
>    fronted or not.
> 
>    2a) A page cache fronted page which has been pinned by GUP long term can use a
>    bounce buffer to allow the file system to write back snap shots of the page.
>    This is handled by the FS recognizing the GUP long term pin and making a copy
>    of the page to be written back.
> 	NOTE: this patch set does not address this path.
> 
>    2b) A FS "controlled" page which is not page cache fronted is either easier
>    to deal with or harder depending on the operation the filesystem is trying
>    to do.
> 
> 	2ba) [Hard case] If the FS operation _is_ a truncate or hole punch the
> 	FS can no longer use the pages in question until the pin has been
> 	removed.  This patch set presents a solution to this by introducing
> 	some reasonable restrictions on user space applications.
> 
> 	2bb) [Easy case] If the FS operation is _not_ a truncate or hole punch
> 	then there is nothing which need be done.  Data is Read or Written
> 	directly to the page.  This is an easy case which would currently work
> 	if not for GUP long term pins being disabled.  Therefore this patch set
> 	need not change access to the file data but does allow for GUP pins
> 	after 2ba above is dealt with.
> 
> 
> This patch series and presents a solution for problem 2ba)
> 
> [1] https://github.com/johnhubbard/linux/tree/gup_dma_core
> 
> [2] ext4/dev branch:
> 
> - https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/log/?h=dev
> 
> 	Specific patches:
> 
> 	[2a] ext4: wait for outstanding dio during truncate in nojournal mode
> 
> 	- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=82a25b027ca48d7ef197295846b352345853dfa8
> 
> 	[2b] ext4: do not delete unlinked inode from orphan list on failed truncate
> 
> 	- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=ee0ed02ca93ef1ecf8963ad96638795d55af2c14
> 
> 	[2c] ext4: gracefully handle ext4_break_layouts() failure during truncate
> 
> 	- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=b9c1c26739ec2d4b4fb70207a0a9ad6747e43f4c
> 
> [3] The definition of long time is debatable but it has been established
> that RDMAs use of pages, minutes or hours after the pin is the extreme case
> which makes this problem most severe.
> 
> 
> Ira Weiny (10):
>   fs/locks: Add trace_leases_conflict
>   fs/locks: Export F_LAYOUT lease to user space
>   mm/gup: Pass flags down to __gup_device_huge* calls
>   mm/gup: Ensure F_LAYOUT lease is held prior to GUP'ing pages
>   fs/ext4: Teach ext4 to break layout leases
>   fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range
>   fs/ext4: Fail truncate if pages are GUP pinned
>   fs/xfs: Teach xfs to use new dax_layout_busy_page()
>   fs/xfs: Fail truncate if pages are GUP pinned
>   mm/gup: Remove FOLL_LONGTERM DAX exclusion
> 
>  fs/Kconfig                       |   1 +
>  fs/dax.c                         |  38 ++++++---
>  fs/ext4/ext4.h                   |   2 +-
>  fs/ext4/extents.c                |   6 +-
>  fs/ext4/inode.c                  |  26 +++++--
>  fs/locks.c                       |  97 ++++++++++++++++++++---
>  fs/xfs/xfs_file.c                |  24 ++++--
>  fs/xfs/xfs_inode.h               |   5 +-
>  fs/xfs/xfs_ioctl.c               |  15 +++-
>  fs/xfs/xfs_iops.c                |  14 +++-
>  fs/xfs/xfs_pnfs.c                |  14 ++--
>  include/linux/dax.h              |   9 ++-
>  include/linux/fs.h               |   2 +-
>  include/linux/mm.h               |   2 +
>  include/trace/events/filelock.h  |  35 +++++++++
>  include/uapi/asm-generic/fcntl.h |   3 +
>  mm/gup.c                         | 129 ++++++++++++-------------------
>  mm/huge_memory.c                 |  12 +++
>  18 files changed, 299 insertions(+), 135 deletions(-)
>