Re: [PATCH] dax: Fix Xarray conversion of dax_unlock_mapping_entry()

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Dec 4, 2018 at 5:35 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Dec 03, 2018 at 07:33:43PM -0800, Dan Williams wrote:
> > On Fri, Nov 30, 2018 at 12:05 PM Dan Williams <dan.j.williams@intel.com> wrote:
> > > > -void dax_unlock_mapping_entry(struct page *page)
> > > > +void dax_unlock_mapping_entry(struct page *page, dax_entry_t entry)
> > >
> > > Let's not require the page to be passed back, it can be derived:
> > >
> > >     page = pfn_to_page(dax_to_pfn((void*) entry));
> > >
> > > A bit more symmetric that way and canonical with other locking schemes
> > > that return a cookie.
> >
> > The patch does not apply on top of the pending fixes. could resend on
> > top of the current libnvdimm-fixes branch [1]?
> >
> > I think because we are changing the calling convention to take return
> > a locked dax_entry_t type, I feel like we should go back to the
> > originally proposed names of these interfaces. I.e.
> >
> >     dax_entry_t dax_lock_page(struct page *page)
> >
> >     void dax_unlock_page(dax_entry_t entry)
> >
> > Yes, it introduces an entry-to-pfn and pfn-to-page conversion.
> > However, If I can't convince you to drop the page argument, lets at
> > least do the name change. I.e. offload the responsibility of conveying
> > that this is not the traditional page lock to the fact that a
> > dax_entry is returned and passed back to the unlock routine.
>
> From: Matthew Wilcox <willy@infradead.org>
> Date: Fri, 30 Nov 2018 11:05:06 -0500
> Subject: [PATCH] dax: Change lock/unlock API
>
> Return the unlock cookie from dax_lock_page() and pass it to
> dax_unlock_page().  This fixes a bug where dax_unlock_page() was
> assuming that the page was PMD-aligned if the entry was a PMD entry.
>
> Debugged-by: Dan Williams <dan.j.williams@intel.com>
> Fixes: 9f32d221301c ("dax: Convert dax_lock_mapping_entry to XArray")
> Signed-off-by: Matthew Wilcox <willy@infradead.org>

Looks good, passes testing. I did fix up the changelog a bit to make
it clearer that this is a fix, not new development, and included more
details about the failing signature from my initial fix. No code
changes. I'll push it out to libnvdimm-fixes for -next to pick up.

---

dax: Fix unlock mismatch with updated API

Internal to dax_unlock_mapping_entry(), dax_unlock_entry() is used to
store a replacement entry in the Xarray at the given xas-index with the
DAX_LOCKED bit clear. When called, dax_unlock_entry() expects the unlocked
value of the entry relative to the current Xarray state to be specified.

In most contexts dax_unlock_entry() is operating in the same scope as
the matched dax_lock_entry(). However, in the dax_unlock_mapping_entry()
case the implementation needs to recall the original entry. In the case
where the original entry is a 'pmd' entry it is possible that the pfn
performed to do the lookup is misaligned to the value retrieved in the
Xarray.

Change the api to return return the unlock cookie from dax_lock_page()
and pass it to dax_unlock_page(). This fixes a bug where
dax_unlock_page() was assuming that the page was PMD-aligned if the
entry was a PMD entry with signatures like:

 WARNING: CPU: 38 PID: 1396 at fs/dax.c:340 dax_insert_entry+0x2b2/0x2d0
 RIP: 0010:dax_insert_entry+0x2b2/0x2d0
 [..]
 Call Trace:
  dax_iomap_pte_fault.isra.41+0x791/0xde0
  ext4_dax_huge_fault+0x16f/0x1f0
  ? up_read+0x1c/0xa0
  __do_fault+0x1f/0x160
  __handle_mm_fault+0x1033/0x1490
  handle_mm_fault+0x18b/0x3d0

Link: https://lkml.kernel.org/r/20181130154902.GL10377@bombadil.infradead.org
Fixes: 9f32d221301c ("dax: Convert dax_lock_mapping_entry to XArray")
Reported-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

---

> ---
> diff --git a/fs/dax.c b/fs/dax.c
> index 3f592dc18d67..48132eca3761 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -379,20 +379,20 @@ static struct page *dax_busy_page(void *entry)
>   * @page: The page whose entry we want to lock
>   *
>   * Context: Process context.
> - * Return: %true if the entry was locked or does not need to be locked.
> + * Return: A cookie to pass to dax_unlock_page() or 0 if the entry could
> + * not be locked.
>   */
> -bool dax_lock_mapping_entry(struct page *page)
> +dax_entry_t dax_lock_page(struct page *page)
>  {
>         XA_STATE(xas, NULL, 0);
>         void *entry;
> -       bool locked;
>
>         /* Ensure page->mapping isn't freed while we look at it */
>         rcu_read_lock();
>         for (;;) {
>                 struct address_space *mapping = READ_ONCE(page->mapping);
>
> -               locked = false;
> +               entry = NULL;
>                 if (!mapping || !dax_mapping(mapping))
>                         break;
>
> @@ -403,7 +403,7 @@ bool dax_lock_mapping_entry(struct page *page)
>                  * otherwise we would not have a valid pfn_to_page()
>                  * translation.
>                  */
> -               locked = true;
> +               entry = (void *)~0UL;
>                 if (S_ISCHR(mapping->host->i_mode))
>                         break;
>
> @@ -426,23 +426,18 @@ bool dax_lock_mapping_entry(struct page *page)
>                 break;
>         }
>         rcu_read_unlock();
> -       return locked;
> +       return (dax_entry_t)entry;
>  }
>
> -void dax_unlock_mapping_entry(struct page *page)
> +void dax_unlock_page(struct page *page, dax_entry_t cookie)
>  {
>         struct address_space *mapping = page->mapping;
>         XA_STATE(xas, &mapping->i_pages, page->index);
> -       void *entry;
>
>         if (S_ISCHR(mapping->host->i_mode))
>                 return;
>
> -       rcu_read_lock();
> -       entry = xas_load(&xas);
> -       rcu_read_unlock();
> -       entry = dax_make_entry(page_to_pfn_t(page), dax_is_pmd_entry(entry));
> -       dax_unlock_entry(&xas, entry);
> +       dax_unlock_entry(&xas, (void *)cookie);
>  }
>
>  /*
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 450b28db9533..0dd316a74a29 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -7,6 +7,8 @@
>  #include <linux/radix-tree.h>
>  #include <asm/pgtable.h>
>
> +typedef unsigned long dax_entry_t;
> +
>  struct iomap_ops;
>  struct dax_device;
>  struct dax_operations {
> @@ -88,8 +90,8 @@ int dax_writeback_mapping_range(struct address_space *mapping,
>                 struct block_device *bdev, struct writeback_control *wbc);
>
>  struct page *dax_layout_busy_page(struct address_space *mapping);
> -bool dax_lock_mapping_entry(struct page *page);
> -void dax_unlock_mapping_entry(struct page *page);
> +dax_entry_t dax_lock_page(struct page *page);
> +void dax_unlock_page(struct page *page, dax_entry_t cookie);
>  #else
>  static inline bool bdev_dax_supported(struct block_device *bdev,
>                 int blocksize)
> @@ -122,14 +124,14 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping,
>         return -EOPNOTSUPP;
>  }
>
> -static inline bool dax_lock_mapping_entry(struct page *page)
> +static inline dax_entry_t dax_lock_page(struct page *page)
>  {
>         if (IS_DAX(page->mapping->host))
> -               return true;
> -       return false;
> +               return ~0UL;
> +       return 0;
>  }
>
> -static inline void dax_unlock_mapping_entry(struct page *page)
> +static inline void dax_unlock_page(struct page *page, dax_entry_t cookie)
>  {
>  }
>  #endif
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 0cd3de3550f0..7c72f2a95785 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1161,6 +1161,7 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>         LIST_HEAD(tokill);
>         int rc = -EBUSY;
>         loff_t start;
> +       dax_entry_t cookie;
>
>         /*
>          * Prevent the inode from being freed while we are interrogating
> @@ -1169,7 +1170,8 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>          * also prevents changes to the mapping of this pfn until
>          * poison signaling is complete.
>          */
> -       if (!dax_lock_mapping_entry(page))
> +       cookie = dax_lock_page(page);
> +       if (!cookie)
>                 goto out;
>
>         if (hwpoison_filter(page)) {
> @@ -1220,7 +1222,7 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>         kill_procs(&tokill, flags & MF_MUST_KILL, !unmap_success, pfn, flags);
>         rc = 0;
>  unlock:
> -       dax_unlock_mapping_entry(page);
> +       dax_unlock_page(page, cookie);
>  out:
>         /* drop pgmap ref acquired in caller */
>         put_dev_pagemap(pgmap);