Re: [patch 1/3] mm: protect set_page_dirty() from ongoing truncation

[Johannes Weiner]

On Tue, Dec 02, 2014 at 10:12:12AM +0100, Jan Kara wrote:
> On Mon 01-12-14 17:58:00, Johannes Weiner wrote:
> > Tejun, while reviewing the code, spotted the following race condition
> > between the dirtying and truncation of a page:
> > 
> > __set_page_dirty_nobuffers()       __delete_from_page_cache()
> >   if (TestSetPageDirty(page))
> >                                      page->mapping = NULL
> > 				     if (PageDirty())
> > 				       dec_zone_page_state(page, NR_FILE_DIRTY);
> > 				       dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> >     if (page->mapping)
> >       account_page_dirtied(page)
> >         __inc_zone_page_state(page, NR_FILE_DIRTY);
> > 	__inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> > 
> > which results in an imbalance of NR_FILE_DIRTY and BDI_RECLAIMABLE.
> > 
> > Dirtiers usually lock out truncation, either by holding the page lock
> > directly, or in case of zap_pte_range(), by pinning the mapcount with
> > the page table lock held.  The notable exception to this rule, though,
> > is do_wp_page(), for which this race exists.  However, do_wp_page()
> > already waits for a locked page to unlock before setting the dirty
> > bit, in order to prevent a race where clear_page_dirty() misses the
> > page bit in the presence of dirty ptes.  Upgrade that wait to a fully
> > locked set_page_dirty() to also cover the situation explained above.
> > 
> > Afterwards, the code in set_page_dirty() dealing with a truncation
> > race is no longer needed.  Remove it.
> > 
> > Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/writeback.h |  1 -
> >  mm/memory.c               | 26 ++++++++++++++++----------
> >  mm/page-writeback.c       | 43 ++++++++++++-------------------------------
> >  3 files changed, 28 insertions(+), 42 deletions(-)
> > 
> > diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> > index a219be961c0a..00048339c23e 100644
> > --- a/include/linux/writeback.h
> > +++ b/include/linux/writeback.h
> > @@ -177,7 +177,6 @@ int write_cache_pages(struct address_space *mapping,
> >  		      struct writeback_control *wbc, writepage_t writepage,
> >  		      void *data);
> >  int do_writepages(struct address_space *mapping, struct writeback_control *wbc);
> > -void set_page_dirty_balance(struct page *page);
> >  void writeback_set_ratelimit(void);
> >  void tag_pages_for_writeback(struct address_space *mapping,
> >  			     pgoff_t start, pgoff_t end);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 3e503831e042..73220eb6e9e3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2150,17 +2150,23 @@ reuse:
> >  		if (!dirty_page)
> >  			return ret;
> >  
> > -		/*
> > -		 * Yes, Virginia, this is actually required to prevent a race
> > -		 * with clear_page_dirty_for_io() from clearing the page dirty
> > -		 * bit after it clear all dirty ptes, but before a racing
> > -		 * do_wp_page installs a dirty pte.
> > -		 *
> > -		 * do_shared_fault is protected similarly.
> > -		 */
> >  		if (!page_mkwrite) {
> > -			wait_on_page_locked(dirty_page);
> > -			set_page_dirty_balance(dirty_page);
> > +			struct address_space *mapping;
> > +			int dirtied;
> > +
> > +			lock_page(dirty_page);
> > +			dirtied = set_page_dirty(dirty_page);
> > +			mapping = dirty_page->mapping;
> > +			unlock_page(dirty_page);
> > +
> > +			if (dirtied && mapping) {
> > +				/*
> > +				 * Some device drivers do not set page.mapping
> > +				 * but still dirty their pages
> > +				 */
>   The comment doesn't make sense to me here. Is it meant to explain why we
> check 'mapping' in the above condition? I always thought truncate is the
> main reason.

Yes, I just copied it from the page_mkwrite case a few lines down, and
there is another copy of it in do_shared_fault().  Truncate is also a
possibility during a race, of course, but even without it we expect
that the mapping can be NULL for certain device drivers.
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