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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id k21sm98571ots.53.2021.08.18.09.35.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Aug 2021 09:35:07 -0700 (PDT) Date: Wed, 18 Aug 2021 09:34:51 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: "Matthew Wilcox (Oracle)" cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm: Remove bogus VM_BUG_ON In-Reply-To: <20210818144932.940640-1-willy@infradead.org> Message-ID: <2197941-297c-f820-aa57-fb5167794fb1@google.com> References: <20210818144932.940640-1-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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) > --- > 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