From 569d0365f571fa6421a5c80bc30d1b2cdab857fe Mon Sep 17 00:00:00 2001 From: Dan Williams Date: Sat, 14 Oct 2017 11:33:32 -0700 Subject: dax: require 'struct page' by default for filesystem dax If a dax buffer from a device that does not map pages is passed to read(2) or write(2) as a target for direct-I/O it triggers SIGBUS. If gdb attempts to examine the contents of a dax buffer from a device that does not map pages it triggers SIGBUS. If fork(2) is called on a process with a dax mapping from a device that does not map pages it triggers SIGBUS. 'struct page' is required otherwise several kernel code paths break in surprising ways. Disable filesystem-dax on devices that do not map pages. In addition to needing pfn_to_page() to be valid we also require devmap pages. We need this to detect dax pages in the get_user_pages_fast() path and so that we can stop managing the VM_MIXEDMAP flag. For DAX drivers that have not supported get_user_pages() to date we allow them to opt-in to supporting DAX with the CONFIG_FS_DAX_LIMITED configuration option which requires ->direct_access() to return pfn_t_special() pfns. This leaves DAX support in brd disabled and scheduled for removal. Note that when the initial dax support was being merged a few years back there was concern that struct page was unsuitable for use with next generation persistent memory devices. The theoretical concern was that struct page access, being such a hotly used data structure in the kernel, would lead to media wear out. While that was a reasonable conservative starting position it has not held true in practice. We have long since committed to using devm_memremap_pages() to support higher order kernel functionality that needs get_user_pages() and pfn_to_page(). Cc: Jeff Moyer Cc: Ross Zwisler Cc: Benjamin Herrenschmidt Cc: Paul Mackerras Cc: Michael Ellerman Cc: Martin Schwidefsky Cc: Heiko Carstens Reviewed-by: Jan Kara Reviewed-by: Christoph Hellwig Reviewed-by: Gerald Schaefer Signed-off-by: Dan Williams --- fs/Kconfig | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'fs/Kconfig') diff --git a/fs/Kconfig b/fs/Kconfig index 7aee6d699fd6..b40128bf6d1a 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -58,6 +58,13 @@ config FS_DAX_PMD depends on ZONE_DEVICE depends on TRANSPARENT_HUGEPAGE +# Selected by DAX drivers that do not expect filesystem DAX to support +# get_user_pages() of DAX mappings. I.e. "limited" indicates no support +# for fork() of processes with MAP_SHARED mappings or support for +# direct-I/O to a DAX mapping. +config FS_DAX_LIMITED + bool + endif # BLOCK # Posix ACL utility routines -- cgit v1.2.3-55-g7522