Commit 2d6d7f98 authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds
Browse files

mm: protect set_page_dirty() from ongoing truncation 



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: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7a3ef208

include/linux/writeback.h

+0 −1
Original line number Diff line number Diff line
@@ -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);

mm/memory.c

+17 −10
Original line number Diff line number Diff line
@@ -2137,17 +2137,24 @@ reuse: 
		if (!dirty_page)

			return ret;



		if (!page_mkwrite) {

			struct address_space *mapping;

			int dirtied;



			lock_page(dirty_page);

			dirtied = set_page_dirty(dirty_page);

			VM_BUG_ON_PAGE(PageAnon(dirty_page), dirty_page);

			mapping = dirty_page->mapping;

			unlock_page(dirty_page);



			if (dirtied && mapping) {

				/*

		 * 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.

				 * Some device drivers do not set page.mapping

				 * but still dirty their pages

				 */

		if (!page_mkwrite) {

			wait_on_page_locked(dirty_page);

			set_page_dirty_balance(dirty_page);

				balance_dirty_pages_ratelimited(mapping);

			}



			/* file_update_time outside page_lock */

			if (vma->vm_file)

				file_update_time(vma->vm_file);

mm/page-writeback.c

+12 −31
Original line number Diff line number Diff line
@@ -1541,16 +1541,6 @@ pause: 
		bdi_start_background_writeback(bdi);

}



void set_page_dirty_balance(struct page *page)

{

	if (set_page_dirty(page)) {

		struct address_space *mapping = page_mapping(page);



		if (mapping)

			balance_dirty_pages_ratelimited(mapping);

	}

}



static DEFINE_PER_CPU(int, bdp_ratelimits);



/*
@@ -2123,32 +2113,25 @@ EXPORT_SYMBOL(account_page_dirtied); 
 * page dirty in that case, but not all the buffers.  This is a "bottom-up"

 * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.

 *

 * Most callers have locked the page, which pins the address_space in memory.

 * But zap_pte_range() does not lock the page, however in that case the

 * mapping is pinned by the vma's ->vm_file reference.

 *

 * We take care to handle the case where the page was truncated from the

 * mapping by re-checking page_mapping() inside tree_lock.

 * The caller must ensure this doesn't race with truncation.  Most will simply

 * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and

 * the pte lock held, which also locks out truncation.

 */

int __set_page_dirty_nobuffers(struct page *page)

{

	if (!TestSetPageDirty(page)) {

		struct address_space *mapping = page_mapping(page);

		struct address_space *mapping2;

		unsigned long flags;



		if (!mapping)

			return 1;



		spin_lock_irqsave(&mapping->tree_lock, flags);

		mapping2 = page_mapping(page);

		if (mapping2) { /* Race with truncate? */

			BUG_ON(mapping2 != mapping);

		BUG_ON(page_mapping(page) != mapping);

		WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));

		account_page_dirtied(page, mapping);

			radix_tree_tag_set(&mapping->page_tree,

				page_index(page), PAGECACHE_TAG_DIRTY);

		}

		radix_tree_tag_set(&mapping->page_tree, page_index(page),

				   PAGECACHE_TAG_DIRTY);

		spin_unlock_irqrestore(&mapping->tree_lock, flags);

		if (mapping->host) {

			/* !PageAnon && !swapper_space */
@@ -2306,11 +2289,9 @@ int clear_page_dirty_for_io(struct page *page) 
		 * We carefully synchronise fault handlers against

		 * installing a dirty pte and marking the page dirty

		 * at this point.  We do this by having them hold the

		 * page lock at some point after installing their

		 * pte, but before marking the page dirty.

		 * Pages are always locked coming in here, so we get

		 * the desired exclusion. See mm/memory.c:do_wp_page()

		 * for more comments.

		 * page lock while dirtying the page, and pages are

		 * always locked coming in here, so we get the desired

		 * exclusion.

		 */

		if (TestClearPageDirty(page)) {

			dec_zone_page_state(page, NR_FILE_DIRTY);

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