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Date:   Fri, 10 Dec 2021 09:20:03 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        William Kucharski <william.kucharski@oracle.com>,
        Christoph Hellwig <hch@lst.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative()
Message-Id: <20211210092003.cf84354b406a47253afc868c@linux-foundation.org>
In-Reply-To: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com>
References: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com>
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On Wed, 8 Dec 2021 23:19:18 -0800 (PST) Hugh Dickins <hughd@google.com> wrote:

> It is not easily reproducible, but on 5.16-rc I have several times hit
> the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative():
> usually from filemap_get_read_batch() for an ext4 read, yesterday from
> next_uptodate_page() from filemap_map_pages() for a shmem fault.
> 
> That BUG used to be placed where page_ref_add_unless() had succeeded,
> but now it is placed before folio_ref_add_unless() is attempted: that
> is not safe, since it is only the acquired reference which makes the
> page safe from racing THP collapse or split.
> 
> We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu()
> has succeeded; but I don't think it adds much value - just delete it.
> 
> Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()")
> Signed-off-by: Hugh Dickins <hughd@google.com>

I added cc:stable to this.