Re: [PATCH v6 07/26] fs/dax: Ensure all pages are idle prior to filesystem unmount

Alistair Popple
2025-01-13T00:59:27Z

On Fri, Jan 10, 2025 at 08:50:19AM -0800, Darrick J. Wong wrote:
> On Fri, Jan 10, 2025 at 05:00:35PM +1100, Alistair Popple wrote:
> > File systems call dax_break_mapping() prior to reallocating file
> > system blocks to ensure the page is not undergoing any DMA or other
> > accesses. Generally this is needed when a file is truncated to ensure
> > that if a block is reallocated nothing is writing to it. However
> > filesystems currently don't call this when an FS DAX inode is evicted.
> >
> > This can cause problems when the file system is unmounted as a page
> > can continue to be under going DMA or other remote access after
> > unmount. This means if the file system is remounted any truncate or
> > other operation which requires the underlying file system block to be
> > freed will not wait for the remote access to complete. Therefore a
> > busy block may be reallocated to a new file leading to corruption.
> >
> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
> >
> > ---
> >
> > Changes for v5:
> >
> > - Don't wait for pages to be idle in non-DAX mappings
> > ---
> > fs/dax.c | 29 +++++++++++++++++++++++++++++
> > fs/ext4/inode.c | 32 ++++++++++++++------------------
> > fs/xfs/xfs_inode.c | 9 +++++++++
> > fs/xfs/xfs_inode.h | 1 +
> > fs/xfs/xfs_super.c | 18 ++++++++++++++++++
> > include/linux/dax.h | 2 ++
> > 6 files changed, 73 insertions(+), 18 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 7008a73..4e49cc4 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -883,6 +883,14 @@ static int wait_page_idle(struct page *page,
> > TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> > }
> >
> > +static void wait_page_idle_uninterruptible(struct page *page,
> > + void (cb)(struct inode *),
> > + struct inode *inode)
> > +{
> > + ___wait_var_event(page, page_ref_count(page) == 1,
> > + TASK_UNINTERRUPTIBLE, 0, 0, cb(inode));
> > +}
> > +
> > /*
> > * Unmaps the inode and waits for any DMA to complete prior to deleting the
> > * DAX mapping entries for the range.
> > @@ -911,6 +919,27 @@ int dax_break_mapping(struct inode *inode, loff_t start, loff_t end,
> > }
> > EXPORT_SYMBOL_GPL(dax_break_mapping);
> >
> > +void dax_break_mapping_uninterruptible(struct inode *inode,
> > + void (cb)(struct inode *))
> > +{
> > + struct page *page;
> > +
> > + if (!dax_mapping(inode->i_mapping))
> > + return;
> > +
> > + do {
> > + page = dax_layout_busy_page_range(inode->i_mapping, 0,
> > + LLONG_MAX);
> > + if (!page)
> > + break;
> > +
> > + wait_page_idle_uninterruptible(page, cb, inode);
> > + } while (true);
> > +
> > + dax_delete_mapping_range(inode->i_mapping, 0, LLONG_MAX);
> > +}
> > +EXPORT_SYMBOL_GPL(dax_break_mapping_uninterruptible);
> > +
> > /*
> > * Invalidate DAX entry if it is clean.
> > */
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index ee8e83f..fa35161 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -163,6 +163,18 @@ int ext4_inode_is_fast_symlink(struct inode *inode)
> > (inode->i_size < EXT4_N_BLOCKS * 4);
> > }
> >
> > +static void ext4_wait_dax_page(struct inode *inode)
> > +{
> > + filemap_invalidate_unlock(inode->i_mapping);
> > + schedule();
> > + filemap_invalidate_lock(inode->i_mapping);
> > +}
> > +
> > +int ext4_break_layouts(struct inode *inode)
> > +{
> > + return dax_break_mapping_inode(inode, ext4_wait_dax_page);
> > +}
> > +
> > /*
> > * Called at the last iput() if i_nlink is zero.
> > */
> > @@ -181,6 +193,8 @@ void ext4_evict_inode(struct inode *inode)
> >
> > trace_ext4_evict_inode(inode);
> >
> > + dax_break_mapping_uninterruptible(inode, ext4_wait_dax_page);
> > +
> > if (EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL)
> > ext4_evict_ea_inode(inode);
> > if (inode->i_nlink) {
> > @@ -3902,24 +3916,6 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
> > return ret;
> > }
> >
> > -static void ext4_wait_dax_page(struct inode *inode)
> > -{
> > - filemap_invalidate_unlock(inode->i_mapping);
> > - schedule();
> > - filemap_invalidate_lock(inode->i_mapping);
> > -}
> > -
> > -int ext4_break_layouts(struct inode *inode)
> > -{
> > - struct page *page;
> > - int error;
> > -
> > - if (WARN_ON_ONCE(!rwsem_is_locked(&inode->i_mapping->invalidate_lock)))
> > - return -EINVAL;
> > -
> > - return dax_break_mapping_inode(inode, ext4_wait_dax_page);
> > -}
> > -
> > /*
> > * ext4_punch_hole: punches a hole in a file by releasing the blocks
> > * associated with the given offset and length
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index 4410b42..c7ec5ab 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -2997,6 +2997,15 @@ xfs_break_dax_layouts(
> > return dax_break_mapping_inode(inode, xfs_wait_dax_page);
> > }
> >
> > +void
> > +xfs_break_dax_layouts_uninterruptible(
> > + struct inode *inode)
> > +{
> > + xfs_assert_ilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL);
> > +
> > + dax_break_mapping_uninterruptible(inode, xfs_wait_dax_page);
> > +}
> > +
> > int
> > xfs_break_layouts(
> > struct inode *inode,
> > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> > index c4f03f6..613797a 100644
> > --- a/fs/xfs/xfs_inode.h
> > +++ b/fs/xfs/xfs_inode.h
> > @@ -594,6 +594,7 @@ xfs_itruncate_extents(
> > }
> >
> > int xfs_break_dax_layouts(struct inode *inode);
> > +void xfs_break_dax_layouts_uninterruptible(struct inode *inode);
> > int xfs_break_layouts(struct inode *inode, uint *iolock,
> > enum layout_break_reason reason);
> >
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 8524b9d..73ec060 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -751,6 +751,23 @@ xfs_fs_drop_inode(
> > return generic_drop_inode(inode);
> > }
> >
> > +STATIC void
> > +xfs_fs_evict_inode(
> > + struct inode *inode)
> > +{
> > + struct xfs_inode *ip = XFS_I(inode);
> > + uint iolock = XFS_IOLOCK_EXCL | XFS_MMAPLOCK_EXCL;
> > +
> > + if (IS_DAX(inode)) {
> > + xfs_ilock(ip, iolock);
> > + xfs_break_dax_layouts_uninterruptible(inode);
> > + xfs_iunlock(ip, iolock);
>
> If we're evicting the inode, why is it necessary to take i_rwsem and the
> mmap invalidation lock? Shouldn't the evicting thread be the only one
> with access to this inode?

Hmm, good point. I think you're right. I can easily stop taking
XFS_IOLOCK_EXCL. Not taking XFS_MMAPLOCK_EXCL is slightly more difficult because
xfs_wait_dax_page() expects it to be taken. Do you think it is worth creating a
separate callback (xfs_wait_dax_page_unlocked()?) specifically for this path or
would you be happy with a comment explaining why we take the XFS_MMAPLOCK_EXCL
lock here?

- Alistair

> --D
>
> > + }
> > +
> > + truncate_inode_pages_final(&inode->i_data);
> > + clear_inode(inode);
> > +}
> > +
> > static void
> > xfs_mount_free(
> > struct xfs_mount *mp)
> > @@ -1189,6 +1206,7 @@ static const struct super_operations xfs_super_operations = {
> > .destroy_inode = xfs_fs_destroy_inode,
> > .dirty_inode = xfs_fs_dirty_inode,
> > .drop_inode = xfs_fs_drop_inode,
> > + .evict_inode = xfs_fs_evict_inode,
> > .put_super = xfs_fs_put_super,
> > .sync_fs = xfs_fs_sync_fs,
> > .freeze_fs = xfs_fs_freeze,
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index ef9e02c..7c3773f 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -274,6 +274,8 @@ static inline int __must_check dax_break_mapping_inode(struct inode *inode,
> > {
> > return dax_break_mapping(inode, 0, LLONG_MAX, cb);
> > }
> > +void dax_break_mapping_uninterruptible(struct inode *inode,
> > + void (cb)(struct inode *));
> > int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
> > struct inode *dest, loff_t destoff,
> > loff_t len, bool *is_same,
> > --
> > git-series 0.9.1
> >