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

[Hugh Dickins]

On Tue, 5 Oct 2021, Rongwei Wang wrote:

> 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);
> +               }

I won't NAK that patch; but I still believe it's unnecessary, and don't
see how it protects against all the races (collapse_file() does not use
that lock, whereas collapse_file() does use page lock).  And if you're
hoping to fix 5.10, then you will have to backport those invalidate_lock
patches there too (they're really intended to protect hole-punching).

> 
> 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.

Yes, that's exactly what I meant, and thank you for intending to try it.

But if that patch had "no effect", then I think you were not running the
kernel with that patch applied: because it deletes the BUG on line 213
of mm/truncate.c, which is what you reported in the first mail!

Or, is line 213 of mm/truncate.c in your 5.10.46-hugetext+ kernel
something else?  I've been looking at 5.15-rc.

But I wasn't proposing to delete it merely to hide the BUG: as I hope
I explained, we could move it below the page->mapping check, but it
wouldn't really be of any value there since tails have NULL page->mapping
anyway (well, I didn't check first and second tails, maybe mapping gets
reused for some compound page field in those). I was proposing to delete
it because the page->mapping check then weeds out the racy case once
we're holding page lock, without the need for adding anything special.

Hugh