On Fri, 24 Oct 2014 17:20:36 -0400 Matthew Wilcox <[email protected]> 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 <[email protected]> Subject: mm-allow-page-fault-handlers-to-perform-the-cow-fix Cc: Kirill A. Shutemov <[email protected]> Cc: Matthew Wilcox <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
