linux-kernel - Re: [PATCH] mm: Remove bogus VM_BUG

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Date:   Wed, 18 Aug 2021 09:34:51 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm: Remove bogus VM_BUG_ON

On Wed, 18 Aug 2021, Matthew Wilcox (Oracle) wrote:

> It is not safe to check page->index without holding the page lock.
> It can be changed if the page is moved between the swap cache and the
> page cache for a shmem file, for example.  There is a VM_BUG_ON below
> which checks page->index is correct after taking the page lock.
> 
> Cc: stable@...r.kernel.org
> Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")

I don't mind that VM_BUG_ON_PAGE() being removed, but question whether
this Fixes anything, and needs to go to stable. Or maybe it's just that
the shmem example is wrong - moving shmem from page to swap cache does
not change page->index. Or maybe you have later changes in your tree
which change that and do require this. Otherwise, I'll have to worry
why my testing has missed it for six months.

Hugh

> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/filemap.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d1458ecf2f51..34de0b14aaa9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2033,17 +2033,16 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>  	XA_STATE(xas, &mapping->i_pages, start);
>  	struct page *page;
>  
>  	rcu_read_lock();
>  	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
>  		if (!xa_is_value(page)) {
>  			if (page->index < start)
>  				goto put;
> -			VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
>  			if (page->index + thp_nr_pages(page) - 1 > end)
>  				goto put;
>  			if (!trylock_page(page))
>  				goto put;
>  			if (page->mapping != mapping || PageWriteback(page))
>  				goto unlock;
>  			VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),
>  					page);
> -- 
> 2.30.2