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Date: Tue, 9 Dec 2014 19:18:45 +0100
From: Jan Kara <jack@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
Michel Lespinasse <walken@...gle.com>, Jan Kara <jack@...e.cz>,
"Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] mm: protect set_page_dirty() from ongoing truncation
On Fri 05-12-14 09:52:44, 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@...nel.org>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: <stable@...r.kernel.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Looks good. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> include/linux/writeback.h | 1 -
> mm/memory.c | 27 +++++++++++++++++----------
> mm/page-writeback.c | 43 ++++++++++++-------------------------------
> 3 files changed, 29 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..72d998eb0438 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2150,17 +2150,24 @@ 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);
> + VM_BUG_ON_PAGE(PageAnon(dirty_page), 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
> + */
> + balance_dirty_pages_ratelimited(mapping);
> + }
> +
> /* file_update_time outside page_lock */
> if (vma->vm_file)
> file_update_time(vma->vm_file);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 19ceae87522d..437174a2aaa3 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -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);
> - 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);
> - }
> + 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);
> spin_unlock_irqrestore(&mapping->tree_lock, flags);
> if (mapping->host) {
> /* !PageAnon && !swapper_space */
> @@ -2305,12 +2288,10 @@ 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.
> + * at this point. We do this by having them hold the
> + * 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);
> --
> 2.1.3
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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