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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id n17sm938960otl.32.2021.08.19.13.42.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Aug 2021 13:42:59 -0700 (PDT) Date: Thu, 19 Aug 2021 13:42:43 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Matthew Wilcox cc: Hugh Dickins , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: Remove bogus VM_BUG_ON In-Reply-To: Message-ID: <813d476-b6d4-456e-b5c3-5fe0455edeb3@google.com> References: <20210818144932.940640-1-willy@infradead.org> <2197941-297c-f820-aa57-fb5167794fb1@google.com> 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 wrote: > On Wed, Aug 18, 2021 at 09:34:51AM -0700, Hugh Dickins wrote: > > 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. > > I'm sorry, I think you're going to have to worry :-( Indeed, it seems that way; or maybe I can leave this testing to syzbot. > Syzbot found it initially: > > https://lore.kernel.org/linux-mm/0000000000009cfcda05c926b34b@google.com/ Ah, that's useful info. Though I can quite see why you didn't mention that originally: it looks as if syzbot hit a find_lock_entries() crash and an irqstate warning about the same time, and its bisection went off and found the commit that introduced those irqstate warnings: neither the root cause of the irqstate warning, nor the cause of the find_lock_entries() crash which it claims in the Subject. I have briefly tried the C reproducer, but didn't get anything out of it; and suspect it may be a reproducer of the irqstate warning rather than the crash which interests you and me. And I can't tell more from the dump, no dump_page() info is shown, and the "Code:" just points into a function epilog of assorted ud2s. > > and then I hit it today during my testing (which is definitely due to > further changes in my tree). Okay, and it's perfectly reasonable for your tree to make changes which require that VM_BUG_ON_PAGE to be removed. But I do not yet understand why it needs to be removed from the current or stable tree. I don't believe it has anything to do with swap cache. The reproducer is mounting with "huge=within_size", and doing lots of truncation: my supposition is that a shmem THP is being collapsed or split, concurrently with that find_lock_entries(). I don't actually see how that would lead to this VM_BUG_ON_PAGE: I imagine find_get_entry()'s xas_reload check after get_speculative should be good enough - but don't know my way around XArray, so mention this in case it triggers an Aha from you. While there's certainly a sense in which removing the VM_BUG_ON_PAGE removes the root cause of the crash, I don't think we understand what is going on here yet: and therefore I'm reluctant to remove it. But I have not given this issue much time, busy with other stuff. > > I should have added: > > Reported-by: syzbot+c87be4f669d920c76330@syzkaller.appspotmail.com That's fair, it did report it, if confusingly. Hugh (p.s. in parentheses, to minimize confusion from going slightly off-topic, but I think I'd be wrong not to mention a separate issue in this area, with mmotm and linux-next since your folios went in: doesn't happen easily, but I have twice hit the include/linux/pagemap.h:258 VM_BUG_ON_PAGE(PageTail(page), page), in page_cache_add_speculative() - both times when serving filemap_map_pages(). I have not thought about it at all, but expect that when you do, you'll simply decide that one is unsafe and has to be deleted.)