Messages in this thread Patch in this message | | | Date | Tue, 13 Jan 2015 13:58:23 -0500 | From | Matthew Wilcox <> | Subject | Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW |
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On Mon, Jan 12, 2015 at 03:09:35PM -0800, Andrew Morton wrote: > 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???
It's not just DAX ... any fault handler that wants to optimise its COW can use the same technique. I could turn this around and ask the mm people why it is the struct page has to be returned locked; what is it protecting against?
I'm pretty sure the answer is only truncate, and so (as with the previous patch), the read lock is perfectly appropriate.
> 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);
How about: /* * The fault handler has no page to lock, so it * holds i_mmap_lock for read to protect against * truncate. */
> } > 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);
(as Jan already pointed out, the indentation needs to be fixed here anyway)
> } > return ret; > uncharge_out: > _ >
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