linux-kernel - Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX page fault handler

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Date:	Thu, 05 Jun 2014 16:38:34 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, willy@...ux.intel.com
Subject: Re: [PATCH v7 07/22] Replace the XIP page fault handler with the
 DAX page fault handler

On Wed, 2014-05-21 at 14:35 -0600, Toshi Kani wrote:
> On Sun, 2014-03-23 at 15:08 -0400, Matthew Wilcox wrote:
>  :
> > +static int do_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> > +			get_block_t get_block)
> > +{
>  :
> > +	error = dax_get_pfn(inode, &bh, &pfn);
> > +	if (error > 0)
> > +		error = vm_insert_mixed(vma, vaddr, pfn);
> > +	mutex_unlock(&mapping->i_mmap_mutex);
> > +
> > +	if (page) {
> > +		delete_from_page_cache(page);
> > +		unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
> > +							PAGE_CACHE_SIZE, 0);
> > +		unlock_page(page);
> > +		page_cache_release(page);
> 
> Hi Matthew,
> 
> I am seeing a problem in this code path, where it deletes a page cache
> page mapped to a hole. Sometimes, page->_mapcount is 0, not -1, which
> leads __delete_from_page_cache(), called from delete_from_page_cache(),
> to hit the following BUG_ON.
> 
>   BUG_ON(page_mapped(page))
> 
> I suppose such page has a shared mapping. Does this code need to take
> care of replacing shared mappings in such case?

Hi Matthew,

The following change works in my environment.  What do you think? 

Thanks,
-Toshi

---
 fs/dax.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index 2d6b4bc..046c6d6 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -26,6 +26,7 @@
 #include <linux/sched.h>
 #include <linux/uio.h>
 #include <linux/vmstat.h>
+#include <linux/rmap.h>
 
 int dax_clear_blocks(struct inode *inode, sector_t block, long size)
 {
@@ -385,6 +386,8 @@ static int do_dax_fault(struct vm_area_struct *vma,
struct vm_fault *vmf,
 	mutex_unlock(&mapping->i_mmap_mutex);
 
 	if (page) {
+		if (page_mapped(page))
+			try_to_unmap(page, TTU_UNMAP|TTU_IGNORE_ACCESS);
 		delete_from_page_cache(page);
 		unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
 							PAGE_CACHE_SIZE, 0);


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