[PATCH 03/14] mm: Protect operations adding pages to page cache with invalidate_lock

Jan Kara
2021-07-12T16:56:33Z

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.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
---
Documentation/filesystems/locking.rst | 62 +++++++++++++------
fs/inode.c | 2 +
include/linux/fs.h | 33 ++++++++++
mm/filemap.c | 88 +++++++++++++++++++++++----
mm/readahead.c | 2 +
mm/rmap.c | 37 +++++------
mm/truncate.c | 3 +-
7 files changed, 176 insertions(+), 51 deletions(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index cdf15492c699..38a3097b6f1c 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 must 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
@@ -573,6 +576,25 @@ 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. Since VFS acquires mapping->invalidate_lock in
+shared mode when loading pages from disk (filemap_fault(), filemap_read(),
+readahead paths), the fallocate implementation must take the invalidate_lock to
+prevent reloading.
+
+->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. To block changes to file contents via a memory mapping during the
+operation, the filesystem must take mapping->invalidate_lock to coordinate
+with ->page_mkwrite.
+
dquot_operations
================

@@ -630,11 +652,11 @@ pfn_mkwrite: yes
access: yes
============= ========= ===========================

-->fault() is called when a previously not present pte is about
-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
+->fault() is called when a previously not present pte is about 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 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.

@@ -647,12 +669,14 @@ page table entry. Pointer to entry associated with the page is passed in
"pte" field in vm_fault structure. Pointers to entries for other offsets
should be calculated relative to "pte".

-->page_mkwrite() is called when a previously read-only pte is
-about to become writeable. The filesystem again must ensure that there are
-no truncate/invalidate races, and then return with the page locked. If
-the page has been truncated, the filesystem should not look up a new page
-like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
-will cause the VM to retry the fault.
+->page_mkwrite() is called when a previously read-only pte is about to become
+writeable. The filesystem again must ensure that there are no
+truncate/invalidate races or races with operations such as ->remap_file_range
+or ->copy_file_range, and then return with the page locked. Usually
+mapping->invalidate_lock is suitable for proper serialization. If the page has
+been truncated, the filesystem should not look up a new page like the ->fault()
+handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to
+retry the fault.

->pfn_mkwrite() is the same as page_mkwrite but when the pte is
VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
diff --git a/fs/inode.c b/fs/inode.c
index c93500d84264..84c528cd1955 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -190,6 +190,8 @@ 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, "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 640574294216..90a80de37ad4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -436,6 +436,10 @@ 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.
+ * It is also used to block modification of page cache contents through
+ * memory mappings.
* @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 +457,7 @@ 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
@@ -814,6 +819,33 @@ static inline void inode_lock_shared_nested(struct inode *inode, unsigned subcla
down_read_nested(&inode->i_rwsem, subclass);
}

+static inline void filemap_invalidate_lock(struct address_space *mapping)
+{
+ down_write(&mapping->invalidate_lock);
+}
+
+static inline void filemap_invalidate_unlock(struct address_space *mapping)
+{
+ up_write(&mapping->invalidate_lock);
+}
+
+static inline void filemap_invalidate_lock_shared(struct address_space *mapping)
+{
+ down_read(&mapping->invalidate_lock);
+}
+
+static inline int filemap_invalidate_trylock_shared(
+ struct address_space *mapping)
+{
+ return down_read_trylock(&mapping->invalidate_lock);
+}
+
+static inline void filemap_invalidate_unlock_shared(
+ struct address_space *mapping)
+{
+ up_read(&mapping->invalidate_lock);
+}
+
void lock_two_nondirectories(struct inode *, struct inode*);
void unlock_two_nondirectories(struct inode *, struct inode*);

@@ -2487,6 +2519,7 @@ 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 acf20eca2fa4..56988c4e655d 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)
@@ -2368,20 +2370,30 @@ static int filemap_update_page(struct kiocb *iocb,
{
int error;

+ if (iocb->ki_flags & IOCB_NOWAIT) {
+ if (!filemap_invalidate_trylock_shared(mapping))
+ return -EAGAIN;
+ } else {
+ filemap_invalidate_lock_shared(mapping);
+ }
+
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)) {
+ filemap_invalidate_unlock_shared(mapping);
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))
@@ -2392,15 +2404,13 @@ 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:
+ filemap_invalidate_unlock_shared(mapping);
+ if (error == AOP_TRUNCATED_PAGE)
+ put_page(page);
return error;
}

@@ -2415,6 +2425,19 @@ 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.
+ */
+ filemap_invalidate_lock_shared(mapping);
error = add_to_page_cache_lru(page, mapping, index,
mapping_gfp_constraint(mapping, GFP_KERNEL));
if (error == -EEXIST)
@@ -2426,9 +2449,11 @@ static int filemap_create_page(struct file *file,
if (error)
goto error;

+ filemap_invalidate_unlock_shared(mapping);
pagevec_add(pvec, page);
return 0;
error:
+ filemap_invalidate_unlock_shared(mapping);
put_page(page);
return error;
}
@@ -2967,6 +2992,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
pgoff_t max_off;
struct page *page;
vm_fault_t ret = 0;
+ bool mapping_locked = false;

max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
if (unlikely(offset >= max_off))
@@ -2988,15 +3014,30 @@ 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);
+ }
+
+ if (!page) {
retry_find:
+ /*
+ * See comment in filemap_create_page() why we need
+ * invalidate_lock
+ */
+ if (!mapping_locked) {
+ filemap_invalidate_lock_shared(mapping);
+ mapping_locked = true;
+ }
page = pagecache_get_page(mapping, offset,
FGP_CREAT|FGP_FOR_MMAP,
vmf->gfp_mask);
if (!page) {
if (fpin)
goto out_retry;
+ filemap_invalidate_unlock_shared(mapping);
return VM_FAULT_OOM;
}
+ } else if (unlikely(!PageUptodate(page))) {
+ filemap_invalidate_lock_shared(mapping);
+ mapping_locked = true;
}

if (!lock_page_maybe_drop_mmap(vmf, page, &fpin))
@@ -3014,8 +3055,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
* We have a locked page in the page cache, now we need to check
* that it's up-to-date. If not, it is going to be due to an error.
*/
- if (unlikely(!PageUptodate(page)))
+ if (unlikely(!PageUptodate(page))) {
+ /*
+ * The page was in cache and uptodate and now it is not.
+ * Strange but possible since we didn't hold the page lock all
+ * the time. Let's drop everything get the invalidate lock and
+ * try again.
+ */
+ if (!mapping_locked) {
+ unlock_page(page);
+ put_page(page);
+ goto retry_find;
+ }
goto page_not_uptodate;
+ }

/*
* We've made it this far and we had to drop our mmap_lock, now is the
@@ -3026,6 +3079,8 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
unlock_page(page);
goto out_retry;
}
+ if (mapping_locked)
+ filemap_invalidate_unlock_shared(mapping);

/*
* Found the page and have a reference on it.
@@ -3056,6 +3111,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)

if (!error || error == AOP_TRUNCATED_PAGE)
goto retry_find;
+ filemap_invalidate_unlock_shared(mapping);

return VM_FAULT_SIGBUS;

@@ -3067,6 +3123,8 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
*/
if (page)
put_page(page);
+ if (mapping_locked)
+ filemap_invalidate_unlock_shared(mapping);
if (fpin)
fput(fpin);
return ret | VM_FAULT_RETRY;
@@ -3437,6 +3495,8 @@ static struct page *do_read_cache_page(struct address_space *mapping,
*
* If the page does not get brought uptodate, return -EIO.
*
+ * The function expects mapping->invalidate_lock to be already held.
+ *
* Return: up to date page on success, ERR_PTR() on failure.
*/
struct page *read_cache_page(struct address_space *mapping,
@@ -3460,6 +3520,8 @@ EXPORT_SYMBOL(read_cache_page);
*
* If the page does not get brought uptodate, return -EIO.
*
+ * The function expects mapping->invalidate_lock to be already held.
+ *
* Return: up to date page on success, ERR_PTR() on failure.
*/
struct page *read_cache_page_gfp(struct address_space *mapping,
diff --git a/mm/readahead.c b/mm/readahead.c
index d589f147f4c2..41b75d76d36e 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();

+ filemap_invalidate_lock_shared(mapping);
/*
* 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);
+ filemap_invalidate_unlock_shared(mapping);
memalloc_nofs_restore(nofs);
}
EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
diff --git a/mm/rmap.c b/mm/rmap.c
index a8b01929ab2e..86471aacc54a 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 0f9becee9789..44ad5e515140 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -412,7 +412,8 @@ 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
+ * mapping->invalidate_lock.
*
* 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