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commit52f476a323upstream. Jeff discovered that performance improves from ~375K iops to ~519K iops on a simple psync-write fio workload when moving the location of 'struct page' from the default PMEM location to DRAM. This result is surprising because the expectation is that 'struct page' for dax is only needed for third party references to dax mappings. For example, a dax-mapped buffer passed to another system call for direct-I/O requires 'struct page' for sending the request down the driver stack and pinning the page. There is no usage of 'struct page' for first party access to a file via read(2)/write(2) and friends. However, this "no page needed" expectation is violated by CONFIG_HARDENED_USERCOPY and the check_copy_size() performed in copy_from_iter_full_nocache() and copy_to_iter_mcsafe(). The check_heap_object() helper routine assumes the buffer is backed by a slab allocator (DRAM) page and applies some checks. Those checks are invalid, dax pages do not originate from the slab, and redundant, dax_iomap_actor() has already validated that the I/O is within bounds. Specifically that routine validates that the logical file offset is within bounds of the file, then it does a sector-to-pfn translation which validates that the physical mapping is within bounds of the block device. Bypass additional hardened usercopy overhead and call the 'no check' versions of the copy_{to,from}_iter operations directly. Fixes:0aed55af88("x86, uaccess: introduce copy_from_iter_flushcache...") Cc: <stable@vger.kernel.org> Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Christoph Hellwig <hch@lst.de> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Matthew Wilcox <willy@infradead.org> Reported-and-tested-by: Jeff Smits <jeff.smits@intel.com> Acked-by: Kees Cook <keescook@chromium.org> Acked-by: Jan Kara <jack@suse.cz> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.