Re: [PATCH 03/13] mm: Protect operations adding pages to page cache with invalidate_lock

[Jan Kara <jack@suse.cz>]

On Tue 25-05-21 14:01:49, Darrick J. Wong wrote:
> On Tue, May 25, 2021 at 03:50:40PM +0200, Jan Kara wrote:
> > Currently, serializing operations such as page fault, read, or readahead
> > against hole punching is rather difficult. The basic race scheme is
> > like:
> > 
> > fallocate(FALLOC_FL_PUNCH_HOLE)			read / fault / ..
> >   truncate_inode_pages_range()
> > 						  <create pages in page
> > 						   cache here>
> >   <update fs block mapping and free blocks>
> > 
> > Now the problem is in this way read / page fault / readahead can
> > instantiate pages in page cache with potentially stale data (if blocks
> > get quickly reused). Avoiding this race is not simple - page locks do
> > not work because we want to make sure there are *no* pages in given
> > range. inode->i_rwsem does not work because page fault happens under
> > mmap_sem which ranks below inode->i_rwsem. Also using it for reads makes
> > the performance for mixed read-write workloads suffer.
> > 
> > So create a new rw_semaphore in the address_space - invalidate_lock -
> > that protects adding of pages to page cache for page faults / reads /
> > readahead.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  Documentation/filesystems/locking.rst | 64 ++++++++++++++++++--------
> >  fs/inode.c                            |  2 +
> >  include/linux/fs.h                    |  6 +++
> >  mm/filemap.c                          | 65 ++++++++++++++++++++++-----
> >  mm/readahead.c                        |  2 +
> >  mm/rmap.c                             | 37 +++++++--------
> >  mm/truncate.c                         |  3 +-
> >  7 files changed, 129 insertions(+), 50 deletions(-)
> > 
> > diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> > index 4ed2b22bd0a8..af425bef55d3 100644
> > --- a/Documentation/filesystems/locking.rst
> > +++ b/Documentation/filesystems/locking.rst
> > @@ -271,19 +271,19 @@ prototypes::
> >  locking rules:
> >  	All except set_page_dirty and freepage may block
> >  
> > -======================	======================== =========
> > -ops			PageLocked(page)	 i_rwsem
> > -======================	======================== =========
> > +======================	======================== =========	===============
> > +ops			PageLocked(page)	 i_rwsem	invalidate_lock
> > +======================	======================== =========	===============
> >  writepage:		yes, unlocks (see below)
> > -readpage:		yes, unlocks
> > +readpage:		yes, unlocks				shared
> >  writepages:
> >  set_page_dirty		no
> > -readahead:		yes, unlocks
> > -readpages:		no
> > +readahead:		yes, unlocks				shared
> > +readpages:		no					shared
> >  write_begin:		locks the page		 exclusive
> >  write_end:		yes, unlocks		 exclusive
> >  bmap:
> > -invalidatepage:		yes
> > +invalidatepage:		yes					exclusive
> >  releasepage:		yes
> >  freepage:		yes
> >  direct_IO:
> > @@ -378,7 +378,10 @@ keep it that way and don't breed new callers.
> >  ->invalidatepage() is called when the filesystem must attempt to drop
> >  some or all of the buffers from the page when it is being truncated. It
> >  returns zero on success. If ->invalidatepage is zero, the kernel uses
> > -block_invalidatepage() instead.
> > +block_invalidatepage() instead. The filesystem should exclusively acquire
> 
> s/should/must/ ?  It's not really optional to lock out invalidations
> anymore now that the page cache synchronizes on invalidate_lock, right?

Right, updated.

> > +invalidate_lock before invalidating page cache in truncate / hole punch path
> > +(and thus calling into ->invalidatepage) to block races between page cache
> > +invalidation and page cache filling functions (fault, read, ...).
> >  
> >  ->releasepage() is called when the kernel is about to try to drop the
> >  buffers from the page in preparation for freeing it.  It returns zero to
> > @@ -573,6 +576,27 @@ in sys_read() and friends.
> >  the lease within the individual filesystem to record the result of the
> >  operation
> >  
> > +->fallocate implementation must be really careful to maintain page cache
> > +consistency when punching holes or performing other operations that invalidate
> > +page cache contents. Usually the filesystem needs to call
> > +truncate_inode_pages_range() to invalidate relevant range of the page cache.
> > +However the filesystem usually also needs to update its internal (and on disk)
> > +view of file offset -> disk block mapping. Until this update is finished, the
> > +filesystem needs to block page faults and reads from reloading now-stale page
> > +cache contents from the disk. VFS provides mapping->invalidate_lock for this
> > +and acquires it in shared mode in paths loading pages from disk
> > +(filemap_fault(), filemap_read(), readahead paths). The filesystem is
> > +responsible for taking this lock in its fallocate implementation and generally
> > +whenever the page cache contents needs to be invalidated because a block is
> > +moving from under a page.
> > +
> > +->copy_file_range and ->remap_file_range implementations need to serialize
> > +against modifications of file data while the operation is running. For
> > +blocking changes through write(2) and similar operations inode->i_rwsem can be
> > +used. For blocking changes through memory mapping, the filesystem can use
> > +mapping->invalidate_lock provided it also acquires it in its ->page_mkwrite
> > +implementation.
> 
> Once this patch lands, will there be any filesystems that can skip
> taking invalidate_lock in ->page_mkwrite and not have problems?  Now
> that the address_space has an invalidation lock, everyone is strongly
> incentivized to use it unless they have yet another layer of locks to do
> more or less the same thing, right?

Well, I assume btrfs will want to keep their special extent tree locking
and thus invalidate_lock is not necessary for it strictly speaking.  Also
filesystems supporting only read, write, mmap, truncate (such as udf,
reiserfs, ...) do not really need invalidate_lock (they usually don't
bother with any page_mkwrite helper in fact). So there are going to be
exceptions. I want to add invalidate_lock locking around truncate handling
for these filesystem as well to make locking rules simpler and to be able
to add assertions into VFS helpers. I didn't plan to do this for
.page_mkwrite as there it might actually hurt performance noticeably.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR