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

[Song Liu]

On Wed, Sep 29, 2021 at 5:02 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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;