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

Darrick J. Wong
2025-01-13T02:47:51Z

On Mon, Jan 13, 2025 at 11:47:41AM +1100, Alistair Popple wrote:
> On Fri, Jan 10, 2025 at 08:44:38AM -0800, Darrick J. Wong wrote:
> > 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?
>
> Yes. I went down the rabbit hole of figuring out why this didn't produce a
> compiler warning and forgot to go back and fix it. Thanks.
>
> > > +
> > > + 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?
>
> Because the lock cycling is already happening in the xfs_wait_dax_page()
> callback which is called as part of the retry loop in dax_break_mapping().

Aha, good point.

--D

> > > 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?
>
> Right. Thanks for the review, have fixed for the next respin.
>
> - Alistair
>
> > --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
> > >
>