Re: [PATCH v4 0/1] mm: introduce put_user_page*(), placeholder versions

John Hubbard
2019-03-08T23:22:03Z

On 3/8/19 1:36 PM, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
>
> Hi Andrew and all,
>
> Can we please apply this (destined for 5.2) once the time is right?
> (I see that -mm just got merged into the main tree today.)
>
> We seem to have pretty solid consensus on the concept and details of the
> put_user_pages() approach. Or at least, if we don't, someone please speak
> up now. Christopher Lameter, especially, since you had some concerns
> recently.

I forgot to update the above bit for this v4 patch. In fact, Christopher
already commented on v3. He disagrees that the whole design should be done at
all, and it's a high-level point about kernel requirements. I think that
the "we've been supporting this for years" argument needs to prevail,
though, so I'm still pushing this patch for 5.2, and hoping that
Christopher eventually agrees.

(The changelog is at the end of this cover letter, btw.)

thanks,
--
John Hubbard
NVIDIA

>
> Therefore, here is the first patch--only. This allows us to begin
> converting the get_user_pages() call sites to use put_user_page(), instead
> of put_page(). This is in order to implement tracking of get_user_page()
> pages.
>
> Normally I'd include a user of this code, but in this case, I think we have
> examples of how it will work in the RFC and related discussions [1]. What
> matters more at this point is unblocking the ability to start fixing up
> various subsystems, through git trees other than linux-mm. For example, the
> Infiniband example conversion now needs to pick up some prerequisite
> patches via the RDMA tree. It seems likely that other call sites may need
> similar attention, and so having put_user_pages() available would really
> make this go more quickly.
>
> Previous cover letter follows:
> ==============================
>
> A discussion of the overall problem is below.
>
> As mentioned in patch 0001, the steps are to fix the problem are:
>
> 1) Provide put_user_page*() routines, intended to be used
> for releasing pages that were pinned via get_user_pages*().
>
> 2) Convert all of the call sites for get_user_pages*(), to
> invoke put_user_page*(), instead of put_page(). This involves dozens of
> call sites, and will take some time.
>
> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
> implement tracking of these pages. This tracking will be separate from
> the existing struct page refcounting.
>
> 4) Use the tracking and identification of these pages, to implement
> special handling (especially in writeback paths) when the pages are
> backed by a filesystem.
>
> Overview
> ========
>
> Some kernel components (file systems, device drivers) need to access
> memory that is specified via process virtual address. For a long time, the
> API to achieve that was get_user_pages ("GUP") and its variations. However,
> GUP has critical limitations that have been overlooked; in particular, GUP
> does not interact correctly with filesystems in all situations. That means
> that file-backed memory + GUP is a recipe for potential problems, some of
> which have already occurred in the field.
>
> GUP was first introduced for Direct IO (O_DIRECT), allowing filesystem code
> to get the struct page behind a virtual address and to let storage hardware
> perform a direct copy to or from that page. This is a short-lived access
> pattern, and as such, the window for a concurrent writeback of GUP'd page
> was small enough that there were not (we think) any reported problems.
> Also, userspace was expected to understand and accept that Direct IO was
> not synchronized with memory-mapped access to that data, nor with any
> process address space changes such as munmap(), mremap(), etc.
>
> Over the years, more GUP uses have appeared (virtualization, device
> drivers, RDMA) that can keep the pages they get via GUP for a long period
> of time (seconds, minutes, hours, days, ...). This long-term pinning makes
> an underlying design problem more obvious.
>
> In fact, there are a number of key problems inherent to GUP:
>
> Interactions with file systems
> ==============================
>
> File systems expect to be able to write back data, both to reclaim pages,
> and for data integrity. Allowing other hardware (NICs, GPUs, etc) to gain
> write access to the file memory pages means that such hardware can dirty
> the pages, without the filesystem being aware. This can, in some cases
> (depending on filesystem, filesystem options, block device, block device
> options, and other variables), lead to data corruption, and also to kernel
> bugs of the form:
>
> kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899!
> backtrace:
> ext4_writepage
> __writepage
> write_cache_pages
> ext4_writepages
> do_writepages
> __writeback_single_inode
> writeback_sb_inodes
> __writeback_inodes_wb
> wb_writeback
> wb_workfn
> process_one_work
> worker_thread
> kthread
> ret_from_fork
>
> ...which is due to the file system asserting that there are still buffer
> heads attached:
>
> ({ \
> BUG_ON(!PagePrivate(page)); \
> ((struct buffer_head *)page_private(page)); \
> })
>
> Dave Chinner's description of this is very clear:
>
> "The fundamental issue is that ->page_mkwrite must be called on every
> write access to a clean file backed page, not just the first one.
> How long the GUP reference lasts is irrelevant, if the page is clean
> and you need to dirty it, you must call ->page_mkwrite before it is
> marked writeable and dirtied. Every. Time."
>
> This is just one symptom of the larger design problem: real filesystems
> that actually write to a backing device, do not actually support
> get_user_pages() being called on their pages, and letting hardware write
> directly to those pages--even though that pattern has been going on since
> about 2005 or so.
>
>
> Long term GUP
> =============
>
> Long term GUP is an issue when FOLL_WRITE is specified to GUP (so, a
> writeable mapping is created), and the pages are file-backed. That can lead
> to filesystem corruption. What happens is that when a file-backed page is
> being written back, it is first mapped read-only in all of the CPU page
> tables; the file system then assumes that nobody can write to the page, and
> that the page content is therefore stable. Unfortunately, the GUP callers
> generally do not monitor changes to the CPU pages tables; they instead
> assume that the following pattern is safe (it's not):
>
> get_user_pages()
>
> Hardware can keep a reference to those pages for a very long time,
> and write to it at any time. Because "hardware" here means "devices
> that are not a CPU", this activity occurs without any interaction
> with the kernel's file system code.
>
> for each page
> set_page_dirty
> put_page()
>
> In fact, the GUP documentation even recommends that pattern.
>
> Anyway, the file system assumes that the page is stable (nothing is writing
> to the page), and that is a problem: stable page content is necessary for
> many filesystem actions during writeback, such as checksum, encryption,
> RAID striping, etc. Furthermore, filesystem features like COW (copy on
> write) or snapshot also rely on being able to use a new page for as memory
> for that memory range inside the file.
>
> Corruption during write back is clearly possible here. To solve that, one
> idea is to identify pages that have active GUP, so that we can use a bounce
> page to write stable data to the filesystem. The filesystem would work
> on the bounce page, while any of the active GUP might write to the
> original page. This would avoid the stable page violation problem, but note
> that it is only part of the overall solution, because other problems
> remain.
>
> Other filesystem features that need to replace the page with a new one can
> be inhibited for pages that are GUP-pinned. This will, however, alter and
> limit some of those filesystem features. The only fix for that would be to
> require GUP users to monitor and respond to CPU page table updates.
> Subsystems such as ODP and HMM do this, for example. This aspect of the
> problem is still under discussion.
>
> Direct IO
> =========
>
> Direct IO can cause corruption, if userspace does Direct-IO that writes to
> a range of virtual addresses that are mmap'd to a file. The pages written
> to are file-backed pages that can be under write back, while the Direct IO
> is taking place. Here, Direct IO races with a write back: it calls
> GUP before page_mkclean() has replaced the CPU pte with a read-only entry.
> The race window is pretty small, which is probably why years have gone by
> before we noticed this problem: Direct IO is generally very quick, and
> tends to finish up before the filesystem gets around to do anything with
> the page contents. However, it's still a real problem. The solution is
> to never let GUP return pages that are under write back, but instead,
> force GUP to take a write fault on those pages. That way, GUP will
> properly synchronize with the active write back. This does not change the
> required GUP behavior, it just avoids that race.
>
> Changes since v3:
>
> * Moved put_user_page*() implementation from swap.c to gup.c, as per
> Jerome's review recommendation.
>
> * Updated wording in patch #1 (and in this cover letter) to refer to real
> filesystems with a backing store, as per Christopher Lameter's feedback.
>
> * Rebased to latest linux.git: commit 3601fe43e816 ("Merge tag
> 'gpio-v5.1-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio")
>
> Changes since v2:
>
> * Reduced down to just one patch, in order to avoid dependencies between
> subsystem git repos.
>
> * Rebased to latest linux.git: commit afe6fe7036c6 ("Merge tag
> 'armsoc-late' of git://git.kernel.org/pub/scm/linux/kernel/git/soc/soc")
>
> * Added Ira's review tag, based on
> https://lore.kernel.org/lkml/20190215002312.GC7512@iweiny-DESK2.sc.intel.com/
>
>
> [1] https://lore.kernel.org/r/20190208075649.3025-3-jhubbard@nvidia.com
> (RFC v2: mm: gup/dma tracking)
>
> Cc: Christian Benvenuti <benve@cisco.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Christopher Lameter <cl@linux.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Dennis Dalessandro <dennis.dalessandro@intel.com>
> Cc: Doug Ledford <dledford@redhat.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Mike Marciniszyn <mike.marciniszyn@intel.com>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Tom Talpey <tom@talpey.com>
>
> John Hubbard (1):
> mm: introduce put_user_page*(), placeholder versions
>
> include/linux/mm.h | 24 ++++++++++++++
> mm/gup.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 106 insertions(+)
>