Messages in this thread Patch in this message | | | Date | Mon, 12 Jan 2015 15:09:35 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW |
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On Fri, 24 Oct 2014 17:20:36 -0400 Matthew Wilcox <matthew.r.wilcox@intel.com> 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 <akpm@linux-foundation.org> Subject: mm-allow-page-fault-handlers-to-perform-the-cow-fix
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Matthew Wilcox <matthew.r.wilcox@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> ---
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: _
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