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authorJohn Hubbard <jhubbard@nvidia.com>2019-05-13 17:19:08 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-05-14 09:47:47 -0700
commitfc1d8e7cca2daa18d2fe56b94874848adf89d7f5 (patch)
tree3a0ce5419c03d22146646490608c1e2e630df6b1 /mm/gup.c
parent4eb0716e868eed963967adb0b1b11d9bd8ca1d01 (diff)
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mm: introduce put_user_page*(), placeholder versions
A discussion of the overall problem is below. As mentioned in patch 0001, the steps are to fix the problem are: 1) Provide put_user_page*() routines, intended to be used for releasing pages that were pinned via get_user_pages*(). 2) Convert all of the call sites for get_user_pages*(), to invoke put_user_page*(), instead of put_page(). This involves dozens of call sites, and will take some time. 3) After (2) is complete, use get_user_pages*() and put_user_page*() to implement tracking of these pages. This tracking will be separate from the existing struct page refcounting. 4) Use the tracking and identification of these pages, to implement special handling (especially in writeback paths) when the pages are backed by a filesystem. Overview ======== Some kernel components (file systems, device drivers) need to access memory that is specified via process virtual address. For a long time, the API to achieve that was get_user_pages ("GUP") and its variations. However, GUP has critical limitations that have been overlooked; in particular, GUP does not interact correctly with filesystems in all situations. That means that file-backed memory + GUP is a recipe for potential problems, some of which have already occurred in the field. GUP was first introduced for Direct IO (O_DIRECT), allowing filesystem code to get the struct page behind a virtual address and to let storage hardware perform a direct copy to or from that page. This is a short-lived access pattern, and as such, the window for a concurrent writeback of GUP'd page was small enough that there were not (we think) any reported problems. Also, userspace was expected to understand and accept that Direct IO was not synchronized with memory-mapped access to that data, nor with any process address space changes such as munmap(), mremap(), etc. Over the years, more GUP uses have appeared (virtualization, device drivers, RDMA) that can keep the pages they get via GUP for a long period of time (seconds, minutes, hours, days, ...). This long-term pinning makes an underlying design problem more obvious. In fact, there are a number of key problems inherent to GUP: Interactions with file systems ============================== File systems expect to be able to write back data, both to reclaim pages, and for data integrity. Allowing other hardware (NICs, GPUs, etc) to gain write access to the file memory pages means that such hardware can dirty the pages, without the filesystem being aware. This can, in some cases (depending on filesystem, filesystem options, block device, block device options, and other variables), lead to data corruption, and also to kernel bugs of the form: kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899! backtrace: ext4_writepage __writepage write_cache_pages ext4_writepages do_writepages __writeback_single_inode writeback_sb_inodes __writeback_inodes_wb wb_writeback wb_workfn process_one_work worker_thread kthread ret_from_fork ...which is due to the file system asserting that there are still buffer heads attached: ({ \ BUG_ON(!PagePrivate(page)); \ ((struct buffer_head *)page_private(page)); \ }) Dave Chinner's description of this is very clear: "The fundamental issue is that ->page_mkwrite must be called on every write access to a clean file backed page, not just the first one. How long the GUP reference lasts is irrelevant, if the page is clean and you need to dirty it, you must call ->page_mkwrite before it is marked writeable and dirtied. Every. Time." This is just one symptom of the larger design problem: real filesystems that actually write to a backing device, do not actually support get_user_pages() being called on their pages, and letting hardware write directly to those pages--even though that pattern has been going on since about 2005 or so. Long term GUP ============= Long term GUP is an issue when FOLL_WRITE is specified to GUP (so, a writeable mapping is created), and the pages are file-backed. That can lead to filesystem corruption. What happens is that when a file-backed page is being written back, it is first mapped read-only in all of the CPU page tables; the file system then assumes that nobody can write to the page, and that the page content is therefore stable. Unfortunately, the GUP callers generally do not monitor changes to the CPU pages tables; they instead assume that the following pattern is safe (it's not): get_user_pages() Hardware can keep a reference to those pages for a very long time, and write to it at any time. Because "hardware" here means "devices that are not a CPU", this activity occurs without any interaction with the kernel's file system code. for each page set_page_dirty put_page() In fact, the GUP documentation even recommends that pattern. Anyway, the file system assumes that the page is stable (nothing is writing to the page), and that is a problem: stable page content is necessary for many filesystem actions during writeback, such as checksum, encryption, RAID striping, etc. Furthermore, filesystem features like COW (copy on write) or snapshot also rely on being able to use a new page for as memory for that memory range inside the file. Corruption during write back is clearly possible here. To solve that, one idea is to identify pages that have active GUP, so that we can use a bounce page to write stable data to the filesystem. The filesystem would work on the bounce page, while any of the active GUP might write to the original page. This would avoid the stable page violation problem, but note that it is only part of the overall solution, because other problems remain. Other filesystem features that need to replace the page with a new one can be inhibited for pages that are GUP-pinned. This will, however, alter and limit some of those filesystem features. The only fix for that would be to require GUP users to monitor and respond to CPU page table updates. Subsystems such as ODP and HMM do this, for example. This aspect of the problem is still under discussion. Direct IO ========= Direct IO can cause corruption, if userspace does Direct-IO that writes to a range of virtual addresses that are mmap'd to a file. The pages written to are file-backed pages that can be under write back, while the Direct IO is taking place. Here, Direct IO races with a write back: it calls GUP before page_mkclean() has replaced the CPU pte with a read-only entry. The race window is pretty small, which is probably why years have gone by before we noticed this problem: Direct IO is generally very quick, and tends to finish up before the filesystem gets around to do anything with the page contents. However, it's still a real problem. The solution is to never let GUP return pages that are under write back, but instead, force GUP to take a write fault on those pages. That way, GUP will properly synchronize with the active write back. This does not change the required GUP behavior, it just avoids that race. Details ======= Introduces put_user_page(), which simply calls put_page(). This provides a way to update all get_user_pages*() callers, so that they call put_user_page(), instead of put_page(). Also introduces put_user_pages(), and a few dirty/locked variations, as a replacement for release_pages(), and also as a replacement for open-coded loops that release multiple pages. These may be used for subsequent performance improvements, via batching of pages to be released. This is the first step of fixing a problem (also described in [1] and [2]) with interactions between get_user_pages ("gup") and filesystems. Problem description: let's start with a bug report. Below, is what happens sometimes, under memory pressure, when a driver pins some pages via gup, and then marks those pages dirty, and releases them. Note that the gup documentation actually recommends that pattern. The problem is that the filesystem may do a writeback while the pages were gup-pinned, and then the filesystem believes that the pages are clean. So, when the driver later marks the pages as dirty, that conflicts with the filesystem's page tracking and results in a BUG(), like this one that I experienced: kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899! backtrace: ext4_writepage __writepage write_cache_pages ext4_writepages do_writepages __writeback_single_inode writeback_sb_inodes __writeback_inodes_wb wb_writeback wb_workfn process_one_work worker_thread kthread ret_from_fork ...which is due to the file system asserting that there are still buffer heads attached: ({ \ BUG_ON(!PagePrivate(page)); \ ((struct buffer_head *)page_private(page)); \ }) Dave Chinner's description of this is very clear: "The fundamental issue is that ->page_mkwrite must be called on every write access to a clean file backed page, not just the first one. How long the GUP reference lasts is irrelevant, if the page is clean and you need to dirty it, you must call ->page_mkwrite before it is marked writeable and dirtied. Every. Time." This is just one symptom of the larger design problem: real filesystems that actually write to a backing device, do not actually support get_user_pages() being called on their pages, and letting hardware write directly to those pages--even though that pattern has been going on since about 2005 or so. The steps are to fix it are: 1) (This patch): provide put_user_page*() routines, intended to be used for releasing pages that were pinned via get_user_pages*(). 2) Convert all of the call sites for get_user_pages*(), to invoke put_user_page*(), instead of put_page(). This involves dozens of call sites, and will take some time. 3) After (2) is complete, use get_user_pages*() and put_user_page*() to implement tracking of these pages. This tracking will be separate from the existing struct page refcounting. 4) Use the tracking and identification of these pages, to implement special handling (especially in writeback paths) when the pages are backed by a filesystem. [1] https://lwn.net/Articles/774411/ : "DMA and get_user_pages()" [2] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()" Link: http://lkml.kernel.org/r/20190327023632.13307-2-jhubbard@nvidia.com Signed-off-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> [docs] Reviewed-by: Ira Weiny <ira.weiny@intel.com> Reviewed-by: Jérôme Glisse <jglisse@redhat.com> Reviewed-by: Christoph Lameter <cl@linux.com> Tested-by: Ira Weiny <ira.weiny@intel.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Christoph Hellwig <hch@infradead.org> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Ralph Campbell <rcampbell@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/gup.c')
-rw-r--r--mm/gup.c105
1 files changed, 105 insertions, 0 deletions
diff --git a/mm/gup.c b/mm/gup.c
index 8e0a0a3a2b2d..2c08248d4fa2 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -28,6 +28,111 @@ struct follow_page_context {
unsigned int page_mask;
};
+typedef int (*set_dirty_func_t)(struct page *page);
+
+static void __put_user_pages_dirty(struct page **pages,
+ unsigned long npages,
+ set_dirty_func_t sdf)
+{
+ unsigned long index;
+
+ for (index = 0; index < npages; index++) {
+ struct page *page = compound_head(pages[index]);
+
+ /*
+ * Checking PageDirty at this point may race with
+ * clear_page_dirty_for_io(), but that's OK. Two key cases:
+ *
+ * 1) This code sees the page as already dirty, so it skips
+ * the call to sdf(). That could happen because
+ * clear_page_dirty_for_io() called page_mkclean(),
+ * followed by set_page_dirty(). However, now the page is
+ * going to get written back, which meets the original
+ * intention of setting it dirty, so all is well:
+ * clear_page_dirty_for_io() goes on to call
+ * TestClearPageDirty(), and write the page back.
+ *
+ * 2) This code sees the page as clean, so it calls sdf().
+ * The page stays dirty, despite being written back, so it
+ * gets written back again in the next writeback cycle.
+ * This is harmless.
+ */
+ if (!PageDirty(page))
+ sdf(page);
+
+ put_user_page(page);
+ }
+}
+
+/**
+ * put_user_pages_dirty() - release and dirty an array of gup-pinned pages
+ * @pages: array of pages to be marked dirty and released.
+ * @npages: number of pages in the @pages array.
+ *
+ * "gup-pinned page" refers to a page that has had one of the get_user_pages()
+ * variants called on that page.
+ *
+ * For each page in the @pages array, make that page (or its head page, if a
+ * compound page) dirty, if it was previously listed as clean. Then, release
+ * the page using put_user_page().
+ *
+ * Please see the put_user_page() documentation for details.
+ *
+ * set_page_dirty(), which does not lock the page, is used here.
+ * Therefore, it is the caller's responsibility to ensure that this is
+ * safe. If not, then put_user_pages_dirty_lock() should be called instead.
+ *
+ */
+void put_user_pages_dirty(struct page **pages, unsigned long npages)
+{
+ __put_user_pages_dirty(pages, npages, set_page_dirty);
+}
+EXPORT_SYMBOL(put_user_pages_dirty);
+
+/**
+ * put_user_pages_dirty_lock() - release and dirty an array of gup-pinned pages
+ * @pages: array of pages to be marked dirty and released.
+ * @npages: number of pages in the @pages array.
+ *
+ * For each page in the @pages array, make that page (or its head page, if a
+ * compound page) dirty, if it was previously listed as clean. Then, release
+ * the page using put_user_page().
+ *
+ * Please see the put_user_page() documentation for details.
+ *
+ * This is just like put_user_pages_dirty(), except that it invokes
+ * set_page_dirty_lock(), instead of set_page_dirty().
+ *
+ */
+void put_user_pages_dirty_lock(struct page **pages, unsigned long npages)
+{
+ __put_user_pages_dirty(pages, npages, set_page_dirty_lock);
+}
+EXPORT_SYMBOL(put_user_pages_dirty_lock);
+
+/**
+ * put_user_pages() - release an array of gup-pinned pages.
+ * @pages: array of pages to be marked dirty and released.
+ * @npages: number of pages in the @pages array.
+ *
+ * For each page in the @pages array, release the page using put_user_page().
+ *
+ * Please see the put_user_page() documentation for details.
+ */
+void put_user_pages(struct page **pages, unsigned long npages)
+{
+ unsigned long index;
+
+ /*
+ * TODO: this can be optimized for huge pages: if a series of pages is
+ * physically contiguous and part of the same compound page, then a
+ * single operation to the head page should suffice.
+ */
+ for (index = 0; index < npages; index++)
+ put_user_page(pages[index]);
+}
+EXPORT_SYMBOL(put_user_pages);
+
static struct page *no_page_table(struct vm_area_struct *vma,
unsigned int flags)
{