[PATCH v2 0/6] mm/fs: gup: don't unmap or drop filesystem buffers

john.hubbard
2018-07-02T00:56:48Z

From: John Hubbard <jhubbard@nvidia.com>

This fixes a few problems that came up when using devices (NICs, GPUs,
for example) that want to have direct access to a chunk of system (CPU)
memory, so that they can DMA to/from that memory. Problems [1] come up
if that memory is backed by persistence storage; for example, an ext4
file system. I've been working on several customer bugs that are hitting
this, and this patchset fixes those bugs.

The bugs happen via:

-- get_user_pages() on some ext4-backed pages
-- device does DMA for a while to/from those pages

-- Somewhere in here, some of the pages get disconnected from the
file system, via try_to_unmap() and eventually drop_buffers()

-- device is all done, device driver calls set_page_dirty_locked, then
put_page()

And then at some point, we see a this BUG():

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)); \
})

How to fix this:

If a page is pinned by any of the get_user_page("gup", here) variants, then
there is no need for that page to be on an LRU. So, this patchset removes
such pages from their LRU, thus leaving the page->lru fields *mostly*
available for tracking gup pages. (The lowest bit of page->lru.next is used
as PageTail, and these flags have to be checked when we don't know if it
really is a tail page or not, so avoid that bit.)

After that, the page is reference-counted via page->dma_pinned_count, and
flagged via page->dma_pinned_flags. The PageDmaPinned flag is cleared when
the reference count hits zero, and the reference count is only used when
the flag is set.

All of the above provides a reliable PageDmaPinned flag, which is then used
to decide when to abort or wait for operations such as:

try_to_unmap()
page_mkclean()

In order to handle page_mkclean(), new information had to be plumbed down
from the filesystems, so that page_mkclean can decide whether to skip
dma-pinned pages, or to wait for them.

Thanks to Matthew Wilcox for suggesting re-using page->lru fields for a
new refcount and flag, and to Jan Kara for explaining the rest of the
design details (how to deal with page_mkclean() and try_to_unmap(),
especially). Also thanks to Dan Williams for design advice and DAX,
long-term pinning, and page flag thoughts.

References:

[1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"

Changes since v1:

-- Use page->lru and full reference counting, instead of a single page flag.
-- Proper handling of page_mkclean().

John Hubbard (6):
mm: get_user_pages: consolidate error handling
mm: introduce page->dma_pinned_flags, _count
mm: introduce zone_gup_lock, for dma-pinned pages
mm/fs: add a sync_mode param for clear_page_dirty_for_io()
mm: track gup pages with page->dma_pinned_* fields
mm: page_mkclean, ttu: handle pinned pages

drivers/video/fbdev/core/fb_defio.c | 3 +-
fs/9p/vfs_addr.c | 2 +-
fs/afs/write.c | 6 +-
fs/btrfs/extent_io.c | 14 ++---
fs/btrfs/file.c | 2 +-
fs/btrfs/free-space-cache.c | 2 +-
fs/btrfs/ioctl.c | 2 +-
fs/ceph/addr.c | 4 +-
fs/cifs/cifssmb.c | 3 +-
fs/cifs/file.c | 5 +-
fs/ext4/inode.c | 5 +-
fs/f2fs/checkpoint.c | 4 +-
fs/f2fs/data.c | 2 +-
fs/f2fs/dir.c | 2 +-
fs/f2fs/gc.c | 4 +-
fs/f2fs/inline.c | 2 +-
fs/f2fs/node.c | 10 ++--
fs/f2fs/segment.c | 3 +-
fs/fuse/file.c | 2 +-
fs/gfs2/aops.c | 2 +-
fs/nfs/write.c | 2 +-
fs/nilfs2/page.c | 2 +-
fs/nilfs2/segment.c | 10 ++--
fs/ubifs/file.c | 2 +-
fs/xfs/xfs_aops.c | 2 +-
include/linux/mm.h | 22 ++++++-
include/linux/mm_types.h | 22 +++++--
include/linux/mmzone.h | 7 +++
include/linux/page-flags.h | 50 ++++++++++++++++
include/linux/rmap.h | 4 +-
mm/gup.c | 93 +++++++++++++++++++++++------
mm/memcontrol.c | 7 +++
mm/memory-failure.c | 3 +-
mm/migrate.c | 2 +-
mm/page-writeback.c | 14 +++--
mm/page_alloc.c | 1 +
mm/rmap.c | 71 ++++++++++++++++++++--
mm/swap.c | 48 +++++++++++++++
mm/truncate.c | 3 +-
mm/vmscan.c | 2 +-
40 files changed, 361 insertions(+), 85 deletions(-)

--
2.18.0