Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Rongwei Wang <rongwei.wang@linux.alibaba.com>]

Hi,
I have run our cases these two days to stress test new Patch #1. The new 
Patch #1 mainly add filemap_invalidate_{un}lock before and after 
truncate_pagecache(), basing on original Patch #1. And the crash has not 
happened.

Now, I keep the original Patch #1, then adding the code below which 
suggested by liu song (I'm not sure which one I should add in the next 
version, Suggested-by or Signed-off-by? If you know, please remind me).

-               if (filemap_nr_thps(inode->i_mapping))
+               if (filemap_nr_thps(inode->i_mapping)) {
+                       filemap_invalidate_lock(inode->i_mapping);
                         truncate_pagecache(inode, 0);
+                       filemap_invalidate_unlock(inode->i_mapping);
+               }

And the reason for keeping the original Patch #1 is mainly to fix the 
race between collapse_file and truncate_pagecache. It seems necessary. 
Despite the two-day test, I did not reproduce this race any more.

In addition, I also test the below method:

diff --git a/mm/truncate.c b/mm/truncate.c
index 3f47190f98a8..33604e4ce60a 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -210,8 +210,6 @@ invalidate_complete_page(struct address_space 
*mapping, struct page *page)

  int truncate_inode_page(struct address_space *mapping, struct page *page)
  {
-       VM_BUG_ON_PAGE(PageTail(page), page);
-
         if (page->mapping != mapping)
                 return -EIO;

I am not very sure this VM_BUG_ON_PAGE(PageTail) is what Hugh means. And
the test results show that only removing this VM_BUG_ON_PAGE(PageTail) 
has no effect. So, I still keep the original Patch #1 to fix one race.

I plan to send Patch v3 after receiving your reply.

Thanks!

On 9/30/21 8:41 AM, Song Liu wrote:
> On Wed, Sep 29, 2021 at 5:02 PM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> On Wed, Sep 29, 2021 at 04:41:48PM -0700, Song Liu wrote:
>>> The issue is NOT caused by concurrent khugepaged:collapse_file() and
>>> truncate_pagecache(inode, 0). With some printks, we can see a clear
>>> time gap (>2 second )  between collapse_file() finishes, and
>>> truncate_pagecache() (which crashes soon). Therefore, my earlier
>>> suggestion that adds deny_write_access() to collapse_file() does NOT
>>> work.
>>>
>>> The crash is actually caused by concurrent truncate_pagecache(inode, 0).
>>> If I change the number of write thread in stress_madvise_dso.c to one,
>>> (IOW, one thread_read and one thread_write), I cannot reproduce the
>>> crash anymore.
>>>
>>> I think this means we cannot fix this issue in collapse_file(), because it
>>> finishes long before the crash.
>>
>> Ah!  So are we missing one or more of these locks:
>>
>>          inode_lock(inode);
>>          filemap_invalidate_lock(mapping);
>>
>> in the open path?
> 
> The following fixes the crash in my test. But I am not sure whether this is the
> best fix.
> 
> Rongwei, could you please run more tests on it?
> 
> Thanks,
> Song
> 
> 
> diff --git i/fs/open.c w/fs/open.c
> index daa324606a41f..d13c4668b2e53 100644
> --- i/fs/open.c
> +++ w/fs/open.c
> @@ -856,8 +856,11 @@ static int do_dentry_open(struct file *f,
>                   * of THPs into the page cache will fail.
>                   */
>                  smp_mb();
> -               if (filemap_nr_thps(inode->i_mapping))
> +               if (filemap_nr_thps(inode->i_mapping)) {
> +                       filemap_invalidate_lock(inode->i_mapping);
>                          truncate_pagecache(inode, 0);
> +                       filemap_invalidate_unlock(inode->i_mapping);
> +               }
>          }
> 
>          return 0;
>