Re: [PATCH v6 05/26] fs/dax: Create a common implementation to break DAX layouts

Darrick J. Wong
2025-01-10T16:46:11Z

On Fri, Jan 10, 2025 at 05:00:33PM +1100, Alistair Popple wrote:
> Prior to freeing a block file systems supporting FS DAX must check
> that the associated pages are both unmapped from user-space and not
> undergoing DMA or other access from eg. get_user_pages(). This is
> achieved by unmapping the file range and scanning the FS DAX
> page-cache to see if any pages within the mapping have an elevated
> refcount.
>
> This is done using two functions - dax_layout_busy_page_range() which
> returns a page to wait for the refcount to become idle on. Rather than
> open-code this introduce a common implementation to both unmap and
> wait for the page to become idle.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>

So now that Dan Carpenter has complained, I guess I should look at
this...

> ---
>
> Changes for v5:
>
> - Don't wait for idle pages on non-DAX mappings
>
> Changes for v4:
>
> - Fixed some build breakage due to missing symbol exports reported by
> John Hubbard (thanks!).
> ---
> fs/dax.c | 33 +++++++++++++++++++++++++++++++++
> fs/ext4/inode.c | 10 +---------
> fs/fuse/dax.c | 27 +++------------------------
> fs/xfs/xfs_inode.c | 23 +++++------------------
> fs/xfs/xfs_inode.h | 2 +-
> include/linux/dax.h | 21 +++++++++++++++++++++
> mm/madvise.c | 8 ++++----
> 7 files changed, 68 insertions(+), 56 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index d010c10..9c3bd07 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -845,6 +845,39 @@ int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index)
> return ret;
> }
>
> +static int wait_page_idle(struct page *page,
> + void (cb)(struct inode *),
> + struct inode *inode)
> +{
> + return ___wait_var_event(page, page_ref_count(page) == 1,
> + TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> +}
> +
> +/*
> + * Unmaps the inode and waits for any DMA to complete prior to deleting the
> + * DAX mapping entries for the range.
> + */
> +int dax_break_mapping(struct inode *inode, loff_t start, loff_t end,
> + void (cb)(struct inode *))
> +{
> + struct page *page;
> + int error;
> +
> + if (!dax_mapping(inode->i_mapping))
> + return 0;
> +
> + do {
> + page = dax_layout_busy_page_range(inode->i_mapping, start, end);
> + if (!page)
> + break;
> +
> + error = wait_page_idle(page, cb, inode);
> + } while (error == 0);

You didn't initialize error to 0, so it could be any value. What if
dax_layout_busy_page_range returns null the first time through the loop?

> +
> + return error;
> +}
> +EXPORT_SYMBOL_GPL(dax_break_mapping);
> +
> /*
> * Invalidate DAX entry if it is clean.
> */

<I'm no expert, skipping to xfs>

> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 42ea203..295730a 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -2715,21 +2715,17 @@ xfs_mmaplock_two_inodes_and_break_dax_layout(
> struct xfs_inode *ip2)
> {
> int error;
> - bool retry;
> struct page *page;
>
> if (ip1->i_ino > ip2->i_ino)
> swap(ip1, ip2);
>
> again:
> - retry = false;
> /* Lock the first inode */
> xfs_ilock(ip1, XFS_MMAPLOCK_EXCL);
> - error = xfs_break_dax_layouts(VFS_I(ip1), &retry);
> - if (error || retry) {
> + error = xfs_break_dax_layouts(VFS_I(ip1));
> + if (error) {
> xfs_iunlock(ip1, XFS_MMAPLOCK_EXCL);
> - if (error == 0 && retry)
> - goto again;

Hmm, so the retry loop has moved into xfs_break_dax_layouts, which means
that we no longer cycle the MMAPLOCK. Why was the lock cycling
unnecessary?

> return error;
> }
>
> @@ -2988,19 +2984,11 @@ xfs_wait_dax_page(
>
> int
> xfs_break_dax_layouts(
> - struct inode *inode,
> - bool *retry)
> + struct inode *inode)
> {
> - struct page *page;
> -
> xfs_assert_ilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL);
>
> - page = dax_layout_busy_page(inode->i_mapping);
> - if (!page)
> - return 0;
> -
> - *retry = true;
> - return dax_wait_page_idle(page, xfs_wait_dax_page, inode);
> + return dax_break_mapping_inode(inode, xfs_wait_dax_page);
> }
>
> int
> @@ -3018,8 +3006,7 @@ xfs_break_layouts(
> retry = false;
> switch (reason) {
> case BREAK_UNMAP:
> - error = xfs_break_dax_layouts(inode, &retry);
> - if (error || retry)
> + if (xfs_break_dax_layouts(inode))

dax_break_mapping can return -ERESTARTSYS, right? So doesn't this need
to be:
error = xfs_break_dax_layouts(inode);
if (error)
break;

Hm?

--D

> break;
> fallthrough;
> case BREAK_WRITE:
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 1648dc5..c4f03f6 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -593,7 +593,7 @@ xfs_itruncate_extents(
> return xfs_itruncate_extents_flags(tpp, ip, whichfork, new_size, 0);
> }
>
> -int xfs_break_dax_layouts(struct inode *inode, bool *retry);
> +int xfs_break_dax_layouts(struct inode *inode);
> int xfs_break_layouts(struct inode *inode, uint *iolock,
> enum layout_break_reason reason);
>
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 9b1ce98..f6583d3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -228,6 +228,20 @@ static inline void dax_read_unlock(int id)
> {
> }
> #endif /* CONFIG_DAX */
> +
> +#if !IS_ENABLED(CONFIG_FS_DAX)
> +static inline int __must_check dax_break_mapping(struct inode *inode,
> + loff_t start, loff_t end, void (cb)(struct inode *))
> +{
> + return 0;
> +}
> +
> +static inline void dax_break_mapping_uninterruptible(struct inode *inode,
> + void (cb)(struct inode *))
> +{
> +}
> +#endif
> +
> bool dax_alive(struct dax_device *dax_dev);
> void *dax_get_private(struct dax_device *dax_dev);
> long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
> @@ -251,6 +265,13 @@ vm_fault_t dax_finish_sync_fault(struct vm_fault *vmf,
> int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index);
> int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
> pgoff_t index);
> +int __must_check dax_break_mapping(struct inode *inode, loff_t start,
> + loff_t end, void (cb)(struct inode *));
> +static inline int __must_check dax_break_mapping_inode(struct inode *inode,
> + void (cb)(struct inode *))
> +{
> + return dax_break_mapping(inode, 0, LLONG_MAX, cb);
> +}
> int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
> struct inode *dest, loff_t destoff,
> loff_t len, bool *is_same,
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 49f3a75..1f4c99e 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1063,7 +1063,7 @@ static int guard_install_pud_entry(pud_t *pud, unsigned long addr,
> pud_t pudval = pudp_get(pud);
>
> /* If huge return >0 so we abort the operation + zap. */
> - return pud_trans_huge(pudval) || pud_devmap(pudval);
> + return pud_trans_huge(pudval);
> }
>
> static int guard_install_pmd_entry(pmd_t *pmd, unsigned long addr,
> @@ -1072,7 +1072,7 @@ static int guard_install_pmd_entry(pmd_t *pmd, unsigned long addr,
> pmd_t pmdval = pmdp_get(pmd);
>
> /* If huge return >0 so we abort the operation + zap. */
> - return pmd_trans_huge(pmdval) || pmd_devmap(pmdval);
> + return pmd_trans_huge(pmdval);
> }
>
> static int guard_install_pte_entry(pte_t *pte, unsigned long addr,
> @@ -1183,7 +1183,7 @@ static int guard_remove_pud_entry(pud_t *pud, unsigned long addr,
> pud_t pudval = pudp_get(pud);
>
> /* If huge, cannot have guard pages present, so no-op - skip. */
> - if (pud_trans_huge(pudval) || pud_devmap(pudval))
> + if (pud_trans_huge(pudval))
> walk->action = ACTION_CONTINUE;
>
> return 0;
> @@ -1195,7 +1195,7 @@ static int guard_remove_pmd_entry(pmd_t *pmd, unsigned long addr,
> pmd_t pmdval = pmdp_get(pmd);
>
> /* If huge, cannot have guard pages present, so no-op - skip. */
> - if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval))
> + if (pmd_trans_huge(pmdval))
> walk->action = ACTION_CONTINUE;
>
> return 0;
> --
> git-series 0.9.1
>