From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga14.intel.com ([192.55.52.115]:43936 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752994AbeCaEN1 (ORCPT ); Sat, 31 Mar 2018 00:13:27 -0400 Subject: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings From: Dan Williams To: linux-nvdimm@lists.01.org Cc: Jan Kara , Jeff Moyer , Dave Chinner , Matthew Wilcox , Alexander Viro , "Darrick J. Wong" , Ross Zwisler , Dave Hansen , Andrew Morton , Christoph Hellwig , Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, jack@suse.cz, snitzer@redhat.com Date: Fri, 30 Mar 2018 21:03:30 -0700 Message-ID: <152246901060.36038.4487158506830998280.stgit@dwillia2-desk3.amr.corp.intel.com> In-Reply-To: <152246892890.36038.18436540150980653229.stgit@dwillia2-desk3.amr.corp.intel.com> References: <152246892890.36038.18436540150980653229.stgit@dwillia2-desk3.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Background: get_user_pages() in the filesystem pins file backed memory pages for access by devices performing dma. However, it only pins the memory pages not the page-to-file offset association. If a file is truncated the pages are mapped out of the file and dma may continue indefinitely into a page that is owned by a device driver. This breaks coherency of the file vs dma, but the assumption is that if userspace wants the file-space truncated it does not matter what data is inbound from the device, it is not relevant anymore. The only expectation is that dma can safely continue while the filesystem reallocates the block(s). Problem: This expectation that dma can safely continue while the filesystem changes the block map is broken by dax. With dax the target dma page *is* the filesystem block. The model of leaving the page pinned for dma, but truncating the file block out of the file, means that the filesytem is free to reallocate a block under active dma to another file and now the expected data-incoherency situation has turned into active data-corruption. Solution: Defer all filesystem operations (fallocate(), truncate()) on a dax mode file while any page/block in the file is under active dma. This solution assumes that dma is transient. Cases where dma operations are known to not be transient, like RDMA, have been explicitly disabled via commits like 5f1d43de5416 "IB/core: disable memory registration of filesystem-dax vmas". The dax_layout_busy_page() routine is called by filesystems with a lock held against mm faults (i_mmap_lock) to find pinned / busy dax pages. The process of looking up a busy page invalidates all mappings to trigger any subsequent get_user_pages() to block on i_mmap_lock. The filesystem continues to call dax_layout_busy_page() until it finally returns no more active pages. This approach assumes that the page pinning is transient, if that assumption is violated the system would have likely hung from the uncompleted I/O. Cc: Jan Kara Cc: Jeff Moyer Cc: Dave Chinner Cc: Matthew Wilcox Cc: Alexander Viro Cc: "Darrick J. Wong" Cc: Ross Zwisler Cc: Dave Hansen Cc: Andrew Morton Reported-by: Christoph Hellwig Reviewed-by: Christoph Hellwig Signed-off-by: Dan Williams --- drivers/dax/super.c | 2 + fs/dax.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/dax.h | 25 ++++++++++++++ mm/gup.c | 5 +++ 4 files changed, 123 insertions(+), 1 deletion(-) diff --git a/drivers/dax/super.c b/drivers/dax/super.c index 3bafaddd02f1..91bfc34e3ca7 100644 --- a/drivers/dax/super.c +++ b/drivers/dax/super.c @@ -167,7 +167,7 @@ struct dax_device { #if IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) static void generic_dax_pagefree(struct page *page, void *data) { - /* TODO: wakeup page-idle waiters */ + wake_up_var(&page->_refcount); } struct dax_device *fs_dax_claim(struct dax_device *dax_dev, void *owner) diff --git a/fs/dax.c b/fs/dax.c index a77394fe586e..c01f7989e0aa 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -355,6 +355,19 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping, } } +static struct page *dax_busy_page(void *entry) +{ + unsigned long pfn; + + for_each_mapped_pfn(entry, pfn) { + struct page *page = pfn_to_page(pfn); + + if (page_ref_count(page) > 1) + return page; + } + return NULL; +} + /* * Find radix tree entry at given index. If it points to an exceptional entry, * return it with the radix tree entry locked. If the radix tree doesn't @@ -496,6 +509,85 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index, return entry; } +/** + * dax_layout_busy_page - find first pinned page in @mapping + * @mapping: address space to scan for a page with ref count > 1 + * + * DAX requires ZONE_DEVICE mapped pages. These pages are never + * 'onlined' to the page allocator so they are considered idle when + * page->count == 1. A filesystem uses this interface to determine if + * any page in the mapping is busy, i.e. for DMA, or other + * get_user_pages() usages. + * + * It is expected that the filesystem is holding locks to block the + * establishment of new mappings in this address_space. I.e. it expects + * to be able to run unmap_mapping_range() and subsequently not race + * mapping_mapped() becoming true. It expects that get_user_pages() pte + * walks are performed under rcu_read_lock(). + */ +struct page *dax_layout_busy_page(struct address_space *mapping) +{ + pgoff_t indices[PAGEVEC_SIZE]; + struct page *page = NULL; + struct pagevec pvec; + pgoff_t index, end; + unsigned i; + + /* + * In the 'limited' case get_user_pages() for dax is disabled. + */ + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) + return NULL; + + if (!dax_mapping(mapping) || !mapping_mapped(mapping)) + return NULL; + + pagevec_init(&pvec); + index = 0; + end = -1; + /* + * Flush dax_layout_lock() sections to ensure all possible page + * references have been taken, or otherwise arrange for faults + * to block on the filesystem lock that is taken for + * establishing new mappings. + */ + unmap_mapping_range(mapping, 0, 0, 1); + synchronize_rcu(); + + while (index < end && pagevec_lookup_entries(&pvec, mapping, index, + min(end - index, (pgoff_t)PAGEVEC_SIZE), + indices)) { + for (i = 0; i < pagevec_count(&pvec); i++) { + struct page *pvec_ent = pvec.pages[i]; + void *entry; + + index = indices[i]; + if (index >= end) + break; + + if (!radix_tree_exceptional_entry(pvec_ent)) + continue; + + spin_lock_irq(&mapping->tree_lock); + entry = get_unlocked_mapping_entry(mapping, index, NULL); + if (entry) + page = dax_busy_page(entry); + put_unlocked_mapping_entry(mapping, index, entry); + spin_unlock_irq(&mapping->tree_lock); + if (page) + break; + } + pagevec_remove_exceptionals(&pvec); + pagevec_release(&pvec); + index++; + + if (page) + break; + } + return page; +} +EXPORT_SYMBOL_GPL(dax_layout_busy_page); + static int __dax_invalidate_mapping_entry(struct address_space *mapping, pgoff_t index, bool trunc) { diff --git a/include/linux/dax.h b/include/linux/dax.h index a36b74aa96e8..1b0ad014bc28 100644 --- a/include/linux/dax.h +++ b/include/linux/dax.h @@ -90,6 +90,8 @@ static inline struct dax_device *fs_dax_get_by_host(const char *host) 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); #else static inline int bdev_dax_supported(struct super_block *sb, int blocksize) { @@ -106,6 +108,11 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping, { return -EOPNOTSUPP; } + +static inline struct page *dax_layout_busy_page(struct address_space *mapping) +{ + return NULL; +} #endif #if IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) @@ -113,6 +120,16 @@ struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner); struct dax_device *fs_dax_claim(struct dax_device *dax_dev, void *owner); void __fs_dax_release(struct dax_device *dax_dev, void *owner); void fs_dax_release(struct dax_device *dax_dev, void *owner); + +static inline void dax_layout_lock(void) +{ + rcu_read_lock(); +} + +static inline void dax_layout_unlock(void) +{ + rcu_read_unlock(); +} #else #ifdef CONFIG_BLOCK static inline struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, @@ -142,6 +159,14 @@ static inline struct dax_device *fs_dax_claim(struct dax_device *dax_dev, static inline void __fs_dax_release(struct dax_device *dax_dev, void *owner) { } + +static inline void dax_layout_lock(void) +{ +} + +static inline void dax_layout_unlock(void) +{ +} #endif int dax_read_lock(void); diff --git a/mm/gup.c b/mm/gup.c index 1b46e6e74881..a81efac6983a 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -13,6 +13,7 @@ #include #include #include +#include #include #include @@ -693,7 +694,9 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, if (unlikely(fatal_signal_pending(current))) return i ? i : -ERESTARTSYS; cond_resched(); + dax_layout_lock(); page = follow_page_mask(vma, start, foll_flags, &page_mask); + dax_layout_unlock(); if (!page) { int ret; ret = faultin_page(tsk, vma, start, &foll_flags, @@ -1809,7 +1812,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, if (gup_fast_permitted(start, nr_pages, write)) { local_irq_disable(); + dax_layout_lock(); gup_pgd_range(addr, end, write, pages, &nr); + dax_layout_unlock(); local_irq_enable(); ret = nr; }