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From: Hugh Dickins <hughd@google.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH] mm: Remove bogus VM_BUG_ON
Date: Wed, 18 Aug 2021 09:34:51 -0700 (PDT)	[thread overview]
Message-ID: <2197941-297c-f820-aa57-fb5167794fb1@google.com> (raw)
In-Reply-To: <20210818144932.940640-1-willy@infradead.org>

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@vger.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@infradead.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

  reply	other threads:[~2021-08-18 16:35 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-18 14:49 Matthew Wilcox (Oracle)
2021-08-18 16:34 ` Hugh Dickins [this message]
2021-08-18 16:45   ` Matthew Wilcox
2021-08-19 20:42     ` Hugh Dickins

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