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

On 10/1/21 12:49 AM, Hugh Dickins wrote:

On Thu, 30 Sep 2021, Matthew Wilcox wrote:

quoted

On Wed, Sep 29, 2021 at 10:24:44PM -0700, Hugh Dickins wrote:

quoted

Aside from the above page->index mischeck in find_lock_entries(),
I now think this bug needs nothing more than simply removing the
VM_BUG_ON_PAGE(PageTail(page), page) from truncate_inode_page().

I don't think that's right.  This bug was also observed when calling
truncate(), so there's clearly a situation where two concurrent calls
to truncate_pagecache() leaves a THP in the cache.

I assume you're thinking of one of the fuzzer blkdev ones:
https://lore.kernel.org/linux-mm/CACkBjsbtF_peC7N_4mRfHML_BeiPe+O9DahTfr84puSG_J9rcQ@mail.gmail.com/ (local)
or
https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@mail.gmail.com/ (local)

I haven't started on those ones yet: yes, I imagine one or both of those
will need a further fix (S_ISREG() check somewhere if we're lucky; but
could well be nastier); but for the bug in this thread, I expect
removing the VM_BUG_ON_PAGE(PageTail) to be enough.

Thanks for your advices!

I'm so sorry that delay the test due to my recent vacation. I plan to 
start further test tomorrow. I think removing the 
VM_BUG_ON_PAGE(PageTail) is a good idea, but also think using 
filemap_invalidate_lock is safer and necessary. And of course, this is 
just my own view! So, now, I tend to use filemap_invalidate_lock and
either check page mapping again or remove VM_BUG_ON_PAGE(PageTail).

Anyway, I will run more tests and then give feedback.

Thanks!

If you're thinking of something else, please send a link if you can - thanks.

Hugh