Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock

Jan Kara
2021-05-13T17:45:06Z

On Wed 12-05-21 08:23:45, Darrick J. Wong wrote:
> On Wed, May 12, 2021 at 03:46:11PM +0200, Jan Kara wrote:
> > +->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.
>
> Question: What is the locking order when acquiring the invalidate_lock
> of two different files? Is it the same as i_rwsem (increasing order of
> the struct inode pointer) or is it the same as the XFS MMAPLOCK that is
> being hoisted here (increasing order of i_ino)?
>
> The reason I ask is that remap_file_range has to do that, but I don't
> see any conversions for the xfs_lock_two_inodes(..., MMAPLOCK_EXCL)
> calls in xfs_ilock2_io_mmap in this series.

Good question. Technically, I don't think there's real need to establish a
single ordering because locks among different filesystems are never going
to be acquired together (effectively each lock type is local per sb and we
are free to define an ordering for each lock type differently). But to
maintain some sanity I guess having the same locking order for doublelock
of i_rwsem and invalidate_lock makes sense. Is there a reason why XFS uses
by-ino ordering? So that we don't have to consider two different orders in
xfs_lock_two_inodes()...

Honza

> > +
> > dquot_operations
> > ================
> >
> > @@ -634,9 +658,9 @@ 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"
> > @@ -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..63a814367118 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -190,6 +190,9 @@ 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 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..93bde2741e0e 100644
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -415,7 +415,7 @@ 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
> >
--
Jan Kara <jack@suse.com>
SUSE Labs, CR