From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW Date: Mon, 12 Jan 2015 15:09:35 -0800 Message-ID: <20150112150935.e617603089bc07e68f0e657c@linux-foundation.org> References: <1414185652-28663-1-git-send-email-matthew.r.wilcox@intel.com> <1414185652-28663-5-git-send-email-matthew.r.wilcox@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@linux.intel.com To: Matthew Wilcox Return-path: In-Reply-To: <1414185652-28663-5-git-send-email-matthew.r.wilcox@intel.com> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, 24 Oct 2014 17:20:36 -0400 Matthew Wilcox wrote: > Currently COW of an XIP file is done by first bringing in a read-only > mapping, then retrying the fault and copying the page. It is much more > efficient to tell the fault handler that a COW is being attempted (by > passing in the pre-allocated page in the vm_fault structure), and allow > the handler to perform the COW operation itself. > > The handler cannot insert the page itself if there is already a read-only > mapping at that address, so allow the handler to return VM_FAULT_LOCKED > and set the fault_page to be NULL. This indicates to the MM code that > the i_mmap_mutex is held instead of the page lock. Again, the locking gets a bit subtle. How can we make this clearer to readers of the core code. I had a shot but it's a bit lame - DAX uses i_mmap_lock for what??? If I know that, I'd know whether to have used i_mmap_lock_read() or i_mmap_lock_write() :( From: Andrew Morton Subject: mm-allow-page-fault-handlers-to-perform-the-cow-fix Cc: Kirill A. Shutemov Cc: Matthew Wilcox Signed-off-by: Andrew Morton --- mm/memory.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff -puN include/linux/mm.h~mm-allow-page-fault-handlers-to-perform-the-cow-fix include/linux/mm.h diff -puN mm/memory.c~mm-allow-page-fault-handlers-to-perform-the-cow-fix mm/memory.c --- a/mm/memory.c~mm-allow-page-fault-handlers-to-perform-the-cow-fix +++ a/mm/memory.c @@ -2961,7 +2961,11 @@ static int do_cow_fault(struct mm_struct unlock_page(fault_page); page_cache_release(fault_page); } else { - mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex); + /* + * DAX doesn't have a page to lock, so it uses + * i_mmap_lock() + */ + i_mmap_unlock_read(&vma->vm_file->f_mapping); } goto uncharge_out; } @@ -2973,7 +2977,11 @@ static int do_cow_fault(struct mm_struct unlock_page(fault_page); page_cache_release(fault_page); } else { - mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex); + /* + * DAX doesn't have a page to lock, so it uses + * i_mmap_lock() + */ + i_mmap_unlock_read(&vma->vm_file->f_mapping); } return ret; uncharge_out: _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org