From: Dave Hansen Date: Tue, 8 Sep 2015 22:00:28 +0000 (-0700) Subject: fs: do not prefault sys_write() user buffer pages X-Git-Tag: v4.3-rc1~40^2~96 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=998ef75ddb;p=karo-tx-linux.git fs: do not prefault sys_write() user buffer pages === Short summary ==== iov_iter_fault_in_readable() works around a really rare case and we can avoid the deadlock it addresses in another way: disable page faults and work around copy failures by faulting after the copy in a slow path instead of before in a hot one. I have a little microbenchmark that does repeated, small writes to tmpfs. This patch speeds that micro up by 6.2%. === Long version === When doing a sys_write() we have a source buffer in userspace and then a target file page. If both of those are the same physical page, there is a potential deadlock that we avoid. It would happen something like this: 1. We start the write to the file 2. Allocate page cache page and set it !Uptodate 3. Touch the userspace buffer to copy in the user data 4. Page fault (since source of the write not yet mapped) 5. Page fault code tries to lock the page and deadlocks (more details on this below) To avoid this, we prefault the page to guarantee that this fault does not occur. But, this prefault comes at a cost. It is one of the most expensive things that we do in a hot write() path (especially if we compare it to the read path). It is working around a pretty rare case. To fix this, it's pretty simple. We move the "prefault" code to run after we attempt the copy. We explicitly disable page faults _during_ the copy, detect the copy failure, then execute the "prefault" ouside of where the page lock needs to be held. iov_iter_copy_from_user_atomic() actually already has an implicit pagefault_disable() inside of it (at least on x86), but we add an explicit one. I don't think we can depend on every kmap_atomic() implementation to pagefault_disable() for eternity. =================================================== The stack trace when this happens looks like this: wait_on_page_bit_killable+0xc0/0xd0 __lock_page_or_retry+0x84/0xa0 filemap_fault+0x1ed/0x3d0 __do_fault+0x41/0xc0 handle_mm_fault+0x9bb/0x1210 __do_page_fault+0x17f/0x3d0 do_page_fault+0xc/0x10 page_fault+0x22/0x30 generic_perform_write+0xca/0x1a0 __generic_file_write_iter+0x190/0x1f0 ext4_file_write_iter+0xe9/0x460 __vfs_write+0xaa/0xe0 vfs_write+0xa6/0x1a0 SyS_write+0x46/0xa0 entry_SYSCALL_64_fastpath+0x12/0x6a 0xffffffffffffffff (Note, this does *NOT* happen in practice today because the kmap_atomic() does a pagefault_disable(). The trace above was obtained by taking out the pagefault_disable().) You can trigger the deadlock with this little code snippet: fd = open("foo", O_RDWR); fdmap = mmap(NULL, len, PROT_WRITE|PROT_READ, MAP_SHARED, fd, 0); write(fd, &fdmap[0], 1); Signed-off-by: Dave Hansen Cc: Al Viro Cc: Michal Hocko Cc: Jens Axboe Cc: Tejun Heo Cc: NeilBrown Cc: Matthew Wilcox Cc: Paul Cassella Cc: Greg Thelen Cc: Andi Kleen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/filemap.c b/mm/filemap.c index 1283fc825458..30d69c0c5a38 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2473,21 +2473,6 @@ ssize_t generic_perform_write(struct file *file, iov_iter_count(i)); again: - /* - * Bring in the user page that we will copy from _first_. - * Otherwise there's a nasty deadlock on copying from the - * same page as we're writing to, without it being marked - * up-to-date. - * - * Not only is this an optimisation, but it is also required - * to check that the address is actually valid, when atomic - * usercopies are used, below. - */ - if (unlikely(iov_iter_fault_in_readable(i, bytes))) { - status = -EFAULT; - break; - } - status = a_ops->write_begin(file, mapping, pos, bytes, flags, &page, &fsdata); if (unlikely(status < 0)) @@ -2495,8 +2480,17 @@ again: if (mapping_writably_mapped(mapping)) flush_dcache_page(page); - + /* + * 'page' is now locked. If we are trying to copy from a + * mapping of 'page' in userspace, the copy might fault and + * would need PageUptodate() to complete. But, page can not be + * made Uptodate without acquiring the page lock, which we hold. + * Deadlock. Avoid with pagefault_disable(). Fix up below with + * iov_iter_fault_in_readable(). + */ + pagefault_disable(); copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes); + pagefault_enable(); flush_dcache_page(page); status = a_ops->write_end(file, mapping, pos, bytes, copied, @@ -2519,6 +2513,14 @@ again: */ bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset, iov_iter_single_seg_count(i)); + /* + * This is the fallback to recover if the copy from + * userspace above faults. + */ + if (unlikely(iov_iter_fault_in_readable(i, bytes))) { + status = -EFAULT; + break; + } goto again; } pos += copied;