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

Darrick J. Wong
2021-05-25T21:01:51Z

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?

> +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?

(Setting aside xfs' behavior for the next three patches, obviously)

--D

> +
> dquot_operations
> ================
>
> @@ -630,11 +654,11 @@ 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 +671,14 @@ 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 c3c88fdb9b2a..897238d9f1e0 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
> @@ -2488,6 +2493,7 @@ 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 ba1068a1837f..4d9ec4c6cc34 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 (!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))
> @@ -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:
> + up_read(&mapping->invalidate_lock);
> + 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.
> + */
> + down_read(&mapping->invalidate_lock);
> 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;
>
> + up_read(&mapping->invalidate_lock);
> pagevec_add(pvec, page);
> return 0;
> error:
> + up_read(&mapping->invalidate_lock);
> put_page(page);
> return error;
> }
> @@ -2988,6 +3013,13 @@ 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,
> @@ -2995,6 +3027,7 @@ 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;
> }
> }
> @@ -3035,9 +3068,11 @@ 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;
>
> @@ -3056,6 +3091,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>
> if (!error || error == AOP_TRUNCATED_PAGE)
> goto retry_find;
> + up_read(&mapping->invalidate_lock);
>
> return VM_FAULT_SIGBUS;
>
> @@ -3067,6 +3103,7 @@ 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;
> @@ -3437,6 +3474,8 @@ 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 +3499,8 @@ 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..9785c54107bb 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 a35cbbbded0d..76d33c3b8ae6 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 57a618c4a0d6..d0cc6588aba2 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -415,7 +415,8 @@ 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
>