[PATCH 02/12] mm: Protect operations adding pages to page cache with invalidate_lock

Jan Kara
2021-04-23T17:30:26Z

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>
CC: ceph-devel@vger.kernel.org
CC: Chao Yu <yuchao0@huawei.com>
CC: Damien Le Moal <damien.lemoal@wdc.com>
CC: "Darrick J. Wong" <darrick.wong@oracle.com>
CC: Hugh Dickins <hughd@google.com>
CC: Jaegeuk Kim <jaegeuk@kernel.org>
CC: Jeff Layton <jlayton@kernel.org>
CC: Johannes Thumshirn <jth@kernel.org>
CC: linux-cifs@vger.kernel.org
CC: <linux-ext4@vger.kernel.org>
CC: linux-f2fs-devel@lists.sourceforge.net
CC: <linux-fsdevel@vger.kernel.org>
CC: <linux-mm@kvack.org>
CC: <linux-xfs@vger.kernel.org>
CC: Miklos Szeredi <miklos@szeredi.hu>
CC: Steve French <sfrench@samba.org>
CC: Ted Tso <tytso@mit.edu>
---
Documentation/filesystems/locking.rst | 39 ++++++++++++-----
fs/inode.c | 3 ++
include/linux/fs.h | 4 ++
mm/filemap.c | 61 +++++++++++++++++++++------
mm/readahead.c | 2 +
mm/rmap.c | 37 ++++++++--------
mm/truncate.c | 2 +-
7 files changed, 106 insertions(+), 42 deletions(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index b7dcc86c92a4..7cbf72862832 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -266,19 +266,19 @@ 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:
@@ -373,7 +373,10 @@ 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
+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
@@ -567,6 +570,20 @@ 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.
+
dquot_operations
================

@@ -628,9 +645,9 @@ access: yes
to be faulted in. The filesystem must find and return the page associated
with the passed in "pgoff" in the vm_fault structure. If it is possible that
the page may be truncated and/or invalidated, then the filesystem must lock
-the page, then ensure it is not already truncated (the page lock will block
-subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
-locked. The VM will unlock the page.
+invalidate_lock, then ensure the page is not already truncated (invalidate_lock
+will block subsequent truncate), and then return with VM_FAULT_LOCKED, and the
+page locked. The VM will unlock the page.

->map_pages() is called when VM asks to map easy accessible pages.
Filesystem should find and map pages associated with offsets from "start_pgoff"
diff --git a/fs/inode.c b/fs/inode.c
index a047ab306f9a..43596dd8b61e 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -191,6 +191,9 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);
mapping->private_data = NULL;
mapping->writeback_index = 0;
+ init_rwsem(&mapping->invalidate_lock);
+ lockdep_set_class(&mapping->invalidate_lock,
+ &sb->s_type->invalidate_lock_key);
inode->i_private = NULL;
inode->i_mapping = mapping;
INIT_HLIST_HEAD(&inode->i_dentry); /* buggered by rcu freeing */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ec8f3ddf4a6a..3fca7bf2d0fb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -435,6 +435,8 @@ int pagecache_write_end(struct file *, struct address_space *mapping,
* struct address_space - Contents of a cacheable, mappable object.
* @host: Owner, either the inode or the block_device.
* @i_pages: Cached pages.
+ * @invalidate_lock: Guards coherency between page cache contents and
+ * file offset->disk block mappings in the filesystem during invalidates
* @gfp_mask: Memory allocation flags to use for allocating pages.
* @i_mmap_writable: Number of VM_SHARED mappings.
* @nr_thps: Number of THPs in the pagecache (non-shmem only).
@@ -453,6 +455,7 @@ int pagecache_write_end(struct file *, struct address_space *mapping,
struct address_space {
struct inode *host;
struct xarray i_pages;
+ struct rw_semaphore invalidate_lock;
gfp_t gfp_mask;
atomic_t i_mmap_writable;
#ifdef CONFIG_READ_ONLY_THP_FOR_FS
@@ -2351,6 +2354,7 @@ struct file_system_type {

struct lock_class_key i_lock_key;
struct lock_class_key i_mutex_key;
+ struct lock_class_key invalidate_lock_key;
struct lock_class_key i_mutex_dir_key;
};

diff --git a/mm/filemap.c b/mm/filemap.c
index bd7c50e060a9..9ea8dfb0609c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -77,7 +77,8 @@
* ->i_pages lock
*
* ->i_rwsem
- * ->i_mmap_rwsem (truncate->unmap_mapping_range)
+ * ->invalidate_lock (acquired by fs in truncate path)
+ * ->i_mmap_rwsem (truncate->unmap_mapping_range)
*
* ->mmap_lock
* ->i_mmap_rwsem
@@ -85,7 +86,8 @@
* ->i_pages lock (arch-dependent flush_dcache_mmap_lock)
*
* ->mmap_lock
- * ->lock_page (access_process_vm)
+ * ->invalidate_lock (filemap_fault)
+ * ->lock_page (filemap_fault, access_process_vm)
*
* ->i_rwsem (generic_perform_write)
* ->mmap_lock (fault_in_pages_readable->do_page_fault)
@@ -2276,20 +2278,30 @@ static int filemap_update_page(struct kiocb *iocb,
{
int error;

+ if (iocb->ki_flags & IOCB_NOWAIT) {
+ if (!down_read_trylock(&mapping->invalidate_lock))
+ return -EAGAIN;
+ } else {
+ down_read(&mapping->invalidate_lock);
+ }
+
if (!trylock_page(page)) {
+ error = -EAGAIN;
if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO))
- return -EAGAIN;
+ goto unlock_mapping;
if (!(iocb->ki_flags & IOCB_WAITQ)) {
+ up_read(&mapping->invalidate_lock);
put_and_wait_on_page_locked(page, TASK_KILLABLE);
return AOP_TRUNCATED_PAGE;
}
error = __lock_page_async(page, iocb->ki_waitq);
if (error)
- return error;
+ goto unlock_mapping;
}

+ error = AOP_TRUNCATED_PAGE;
if (!page->mapping)
- goto truncated;
+ goto unlock;

error = 0;
if (filemap_range_uptodate(mapping, iocb->ki_pos, iter, page))
@@ -2300,15 +2312,13 @@ static int filemap_update_page(struct kiocb *iocb,
goto unlock;

error = filemap_read_page(iocb->ki_filp, mapping, page);
- if (error == AOP_TRUNCATED_PAGE)
- put_page(page);
- return error;
-truncated:
- unlock_page(page);
- put_page(page);
- return AOP_TRUNCATED_PAGE;
+ goto unlock_mapping;
unlock:
unlock_page(page);
+unlock_mapping:
+ up_read(&mapping->invalidate_lock);
+ if (error == AOP_TRUNCATED_PAGE)
+ put_page(page);
return error;
}

@@ -2323,6 +2333,19 @@ static int filemap_create_page(struct file *file,
if (!page)
return -ENOMEM;

+ /*
+ * Protect against truncate / hole punch. Grabbing invalidate_lock here
+ * assures we cannot instantiate and bring uptodate new pagecache pages
+ * after evicting page cache during truncate and before actually
+ * freeing blocks. Note that we could release invalidate_lock after
+ * inserting the page into page cache as the locked page would then be
+ * enough to synchronize with hole punching. But there are code paths
+ * such as filemap_update_page() filling in partially uptodate pages or
+ * ->readpages() that need to hold invalidate_lock while mapping blocks
+ * for IO so let's hold the lock here as well to keep locking rules
+ * simple.
+ */
+ down_read(&mapping->invalidate_lock);
error = add_to_page_cache_lru(page, mapping, index,
mapping_gfp_constraint(mapping, GFP_KERNEL));
if (error == -EEXIST)
@@ -2334,9 +2357,11 @@ static int filemap_create_page(struct file *file,
if (error)
goto error;

+ up_read(&mapping->invalidate_lock);
pagevec_add(pvec, page);
return 0;
error:
+ up_read(&mapping->invalidate_lock);
put_page(page);
return error;
}
@@ -2896,6 +2921,13 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
ret = VM_FAULT_MAJOR;
fpin = do_sync_mmap_readahead(vmf);
+ }
+
+ /*
+ * See comment in filemap_create_page() why we need invalidate_lock
+ */
+ down_read(&mapping->invalidate_lock);
+ if (!page) {
retry_find:
page = pagecache_get_page(mapping, offset,
FGP_CREAT|FGP_FOR_MMAP,
@@ -2903,6 +2935,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
if (!page) {
if (fpin)
goto out_retry;
+ up_read(&mapping->invalidate_lock);
return VM_FAULT_OOM;
}
}
@@ -2943,9 +2976,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
if (unlikely(offset >= max_off)) {
unlock_page(page);
put_page(page);
+ up_read(&mapping->invalidate_lock);
return VM_FAULT_SIGBUS;
}

+ up_read(&mapping->invalidate_lock);
vmf->page = page;
return ret | VM_FAULT_LOCKED;

@@ -2971,6 +3006,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
if (!error || error == AOP_TRUNCATED_PAGE)
goto retry_find;

+ up_read(&mapping->invalidate_lock);
shrink_readahead_size_eio(ra);
return VM_FAULT_SIGBUS;

@@ -2982,6 +3018,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
*/
if (page)
put_page(page);
+ up_read(&mapping->invalidate_lock);
if (fpin)
fput(fpin);
return ret | VM_FAULT_RETRY;
diff --git a/mm/readahead.c b/mm/readahead.c
index c5b0457415be..37dd07b32c67 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -192,6 +192,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
*/
unsigned int nofs = memalloc_nofs_save();

+ down_read(&mapping->invalidate_lock);
/*
* Preallocate as many pages as we will need.
*/
@@ -236,6 +237,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
* will then handle the error.
*/
read_pages(ractl, &page_pool, false);
+ up_read(&mapping->invalidate_lock);
memalloc_nofs_restore(nofs);
}
EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
diff --git a/mm/rmap.c b/mm/rmap.c
index dba8cb8a5578..e4f769a4dcc8 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -22,24 +22,25 @@
*
* inode->i_rwsem (while writing or truncating, not reading or faulting)
* mm->mmap_lock
- * page->flags PG_locked (lock_page) * (see hugetlbfs below)
- * hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
- * mapping->i_mmap_rwsem
- * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
- * anon_vma->rwsem
- * mm->page_table_lock or pte_lock
- * swap_lock (in swap_duplicate, swap_info_get)
- * mmlist_lock (in mmput, drain_mmlist and others)
- * mapping->private_lock (in __set_page_dirty_buffers)
- * lock_page_memcg move_lock (in __set_page_dirty_buffers)
- * i_pages lock (widely used)
- * lruvec->lru_lock (in lock_page_lruvec_irq)
- * inode->i_lock (in set_page_dirty's __mark_inode_dirty)
- * bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
- * sb_lock (within inode_lock in fs/fs-writeback.c)
- * i_pages lock (widely used, in set_page_dirty,
- * in arch-dependent flush_dcache_mmap_lock,
- * within bdi.wb->list_lock in __sync_single_inode)
+ * mapping->invalidate_lock (in filemap_fault)
+ * page->flags PG_locked (lock_page) * (see hugetlbfs below)
+ * hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
+ * mapping->i_mmap_rwsem
+ * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
+ * anon_vma->rwsem
+ * mm->page_table_lock or pte_lock
+ * swap_lock (in swap_duplicate, swap_info_get)
+ * mmlist_lock (in mmput, drain_mmlist and others)
+ * mapping->private_lock (in __set_page_dirty_buffers)
+ * lock_page_memcg move_lock (in __set_page_dirty_buffers)
+ * i_pages lock (widely used)
+ * lruvec->lru_lock (in lock_page_lruvec_irq)
+ * inode->i_lock (in set_page_dirty's __mark_inode_dirty)
+ * bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
+ * sb_lock (within inode_lock in fs/fs-writeback.c)
+ * i_pages lock (widely used, in set_page_dirty,
+ * in arch-dependent flush_dcache_mmap_lock,
+ * within bdi.wb->list_lock in __sync_single_inode)
*
* anon_vma->rwsem,mapping->i_mmap_rwsem (memory_failure, collect_procs_anon)
* ->tasklist_lock
diff --git a/mm/truncate.c b/mm/truncate.c
index 2cf71d8c3c62..464ad70a081f 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -416,7 +416,7 @@ EXPORT_SYMBOL(truncate_inode_pages_range);
* @mapping: mapping to truncate
* @lstart: offset from which to truncate
*
- * Called under (and serialised by) inode->i_rwsem.
+ * Called under (and serialised by) inode->i_rwsem and inode->i_mapping_rwsem.
*
* Note: When this function returns, there can be a page in the process of
* deletion (inside __delete_from_page_cache()) in the specified range. Thus
--
2.26.2