On Mon, 23 Mar 2015 14:49:32 +0200 Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote:
> From: Yigal Korman <yigal@xxxxxxxxxxxxx>
>
> This will allow FS that uses VM_PFNMAP | VM_MIXEDMAP (no page structs)
> to get notified when access is a write to a read-only PFN.
>
> This can happen if we mmap() a file then first mmap-read from it
> to page-in a read-only PFN, than we mmap-write to the same page.
>
> We need this functionality to fix a DAX bug, where in the scenario
> above we fail to set ctime/mtime though we modified the file.
> An xfstest is attached to this patchset that shows the failure
> and the fix. (A DAX patch will follow)
>
> This functionality is extra important for us, because upon
> dirtying of a pmem page we also want to RDMA the page to a
> remote cluster node.
>
> We define a new pfn_mkwrite and do not reuse page_mkwrite because
> 1 - The name ;-)
> 2 - But mainly because it would take a very long and tedious
> audit of all page_mkwrite functions of VM_MIXEDMAP/VM_PFNMAP
> users. To make sure they do not now CRASH. For example current
> DAX code (which this is for) would crash.
> If we would want to reuse page_mkwrite, We will need to first
> patch all users, so to not-crash-on-no-page. Then enable this
> patch. But even if I did that I would not sleep so well at night.
> Adding a new vector is the safest thing to do, and is not that
> expensive. an extra pointer at a static function vector per driver.
> Also the new vector is better for performance, because else we
> Will call all current Kernel vectors, so to:
> check-ha-no-page-do-nothing and return.
>
> No need to call it from do_shared_fault because do_wp_page is called to
> change pte permissions anyway.
Looks OK to me.
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1982,6 +1982,22 @@ static int do_page_mkwrite(struct vm_area_struct *vma, struct page *page,
> return ret;
> }
>
> +static int do_pfn_mkwrite(struct vm_area_struct *vma, unsigned long address)
> +{
> + struct vm_fault vmf;
> +
> + if (!vma->vm_ops || !vma->vm_ops->pfn_mkwrite)
> + return 0;
> +
> + vmf.page = 0;
> + vmf.pgoff = (((address & PAGE_MASK) - vma->vm_start) >> PAGE_SHIFT) +
> + vma->vm_pgoff;
> + vmf.virtual_address = (void __user *)(address & PAGE_MASK);
> + vmf.flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
> +
> + return vma->vm_ops->pfn_mkwrite(vma, &vmf);
> +}
It might be a little neater to use
if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
struct vm_fault vmf = {
...
};
...
}
> /*
> * This routine handles present pages, when users try to write
> * to a shared page. It is done by copying the page to a new address
> @@ -2025,8 +2041,17 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> * accounting on raw pfn maps.
> */
> if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> - (VM_WRITE|VM_SHARED))
> + (VM_WRITE|VM_SHARED)) {
> + pte_unmap_unlock(page_table, ptl);
> + ret = do_pfn_mkwrite(vma, address);
> + if (ret & VM_FAULT_ERROR)
> + return ret;
> + page_table = pte_offset_map_lock(mm, pmd, address,
> + &ptl);
> + if (!pte_same(*page_table, orig_pte))
> + goto unlock;
> goto reuse;
> + }
> goto gotten;
There are significant pending changes in this area. See linux-next,
or http://ozlabs.org/~akpm/mmots/broken-out/mm-refactor-*
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